tidy: update html4/sax/test_parser.rb to use minitest::spec (#3285)

**What problem is this PR intended to solve?**

I'm about to do some work on the sax parser encoding functionality, so
let's clean this file up.

test/html4/sax/test_parser.rb

@@ -4,80 +4,77 @@
 require "helper"
 
 module Nokogiri
-  module HTML
-    module SAX
-      class TestParser < Nokogiri::SAX::TestCase
-        def setup
-          super
-          @parser = Nokogiri::HTML4::SAX::Parser.new(Doc.new)
-        end
+  module SAX
+    class TestCase
+      describe Nokogiri::HTML4::SAX::Parser do
+        let(:parser) { Nokogiri::HTML4::SAX::Parser.new(Doc.new) }
 
-        def test_parse_empty_document
+        it "parse_empty_document" do
           # This caused a segfault in libxml 2.6.x
-          assert_nil(@parser.parse(""))
+          assert_nil(parser.parse(""))
         end
 
-        def test_parse_empty_file
+        it "parse_empty_file" do
           # Make sure empty files don't break stuff
           empty_file_name = File.join(ASSETS_DIR, "bogus.xml")
 
           refute_raises do
-            @parser.parse_file(empty_file_name)
+            parser.parse_file(empty_file_name)
           end
         end
 
-        def test_parse_file
-          @parser.parse_file(HTML_FILE)
+        it "parse_file" do
+          parser.parse_file(HTML_FILE)
 
           # Take a look at the comment in test_parse_document to know
           # a possible reason to this difference.
           if Nokogiri.uses_libxml?
-            assert_equal(1111, @parser.document.end_elements.length)
+            assert_equal(1111, parser.document.end_elements.length)
           else
-            assert_equal(1120, @parser.document.end_elements.length)
+            assert_equal(1120, parser.document.end_elements.length)
           end
         end
 
-        def test_parse_file_nil_argument
+        it "parse_file_nil_argument" do
           assert_raises(ArgumentError) do
-            @parser.parse_file(nil)
+            parser.parse_file(nil)
           end
         end
 
-        def test_parse_file_non_existent
+        it "parse_file_non_existent" do
           assert_raises(Errno::ENOENT) do
-            @parser.parse_file("there_is_no_reasonable_way_this_file_exists")
+            parser.parse_file("there_is_no_reasonable_way_this_file_exists")
           end
         end
 
-        def test_parse_file_with_dir
+        it "parse_file_with_dir" do
           assert_raises(Errno::EISDIR) do
-            @parser.parse_file(File.dirname(__FILE__))
+            parser.parse_file(File.dirname(__FILE__))
           end
         end
 
-        def test_parse_memory_nil
+        it "parse_memory_nil" do
           assert_raises(TypeError) do
-            @parser.parse_memory(nil)
+            parser.parse_memory(nil)
           end
         end
 
-        def test_parse_force_encoding
-          @parser.parse_memory(<<-HTML, "UTF-8")
+        it "parse_force_encoding" do
+          parser.parse_memory(<<-HTML, "UTF-8")
           <meta http-equiv="Content-Type" content="text/html; charset=windows-1251">
           Информация
           HTML
           assert_equal(
             "Информация",
-            @parser.document.data.join.strip,
+            parser.document.data.join.strip,
           )
         end
 
-        def test_parse_document
-          @parser.parse_memory(<<-eoxml)
+        it "parse_document" do
+          parser.parse_memory(<<~HTML)
             <p>Paragraph 1</p>
             <p>Paragraph 2</p>
-          eoxml
+          HTML
 
           # JRuby version is different because of the internal implementation
           # JRuby version uses NekoHTML which inserts empty "head" elements.
@@ -88,17 +85,17 @@ def test_parse_document
           if Nokogiri.uses_libxml?
             assert_equal(
               [["html", []], ["body", []], ["p", []], ["p", []]],
-              @parser.document.start_elements,
+              parser.document.start_elements,
             )
           else
             assert_equal(
               [["html", []], ["head", []], ["body", []], ["p", []], ["p", []]],
-              @parser.document.start_elements,
+              parser.document.start_elements,
             )
           end
         end
 
-        def test_parser_attributes
+        it "parser_attributes" do
           html = <<~eohtml
             <html>
               <head>
@@ -112,7 +109,7 @@ def test_parser_attributes
           eohtml
 
           block_called = false
-          @parser.parse(html) do |ctx|
+          parser.parse(html) do |ctx|
             block_called = true
             ctx.replace_entities = true
           end
@@ -136,40 +133,40 @@ def test_parser_attributes
                 ["size", "2"],
               ],],
             ],
-            @parser.document.start_elements,
+            parser.document.start_elements,
           )
         end
 
-        HTML_WITH_BR_TAG = <<-EOF
-        <html>
-          <head></head>
-          <body>
-            <div>
-              hello
-              <br>
-            </div>
-
-            <div>
-              hello again
-            </div>
-          </body>
-        </html>
-        EOF
-
-        def test_parsing_dom_error_from_string
-          @parser.parse(HTML_WITH_BR_TAG)
-          assert_equal(6, @parser.document.start_elements.length)
+        let(:html_with_br_tag) { <<~HTML }
+          <html>
+            <head></head>
+            <body>
+              <div>
+                hello
+                <br>
+              </div>
+
+              <div>
+                hello again
+              </div>
+            </body>
+          </html>
+        HTML
+
+        it "parsing_dom_error_from_string" do
+          parser.parse(html_with_br_tag)
+          assert_equal(6, parser.document.start_elements.length)
         end
 
-        def test_parsing_dom_error_from_io
-          @parser.parse(StringIO.new(HTML_WITH_BR_TAG))
-          assert_equal(6, @parser.document.start_elements.length)
+        it "parsing_dom_error_from_io" do
+          parser.parse(StringIO.new(html_with_br_tag))
+          assert_equal(6, parser.document.start_elements.length)
         end
 
-        def test_empty_processing_instruction
+        it "empty_processing_instruction" do
           # https://github.com/sparklemotion/nokogiri/issues/845
           refute_raises do
-            @parser.parse_memory("<strong>this will segfault<?strong>")
+            parser.parse_memory("<strong>this will segfault<?strong>")
           end
         end