Add inversive test

spec/ruby/language/fixtures/non_frozen_interpolation.rb

@@ -0,0 +1,3 @@
+# frozen_string_literal: false
+
+p "foo #{14}".frozen?

spec/ruby/language/string_spec.rb

@@ -264,6 +264,12 @@ def long_string_literals
     it "interpolated string is frozen" do
       ruby_exe(fixture(__FILE__, "freeze_magic_comment_interpolation.rb")).chomp.should == "true"
     end
+
+    describe "when disabled" do
+      it "interpolated string is not frozen" do
+        ruby_exe(fixture(__FILE__, "non_frozen_interpolation.rb")).chomp.should == "false"
+      end
+    end
   end
 
 end

spec/tags/language/string_tags.txt

@@ -4,3 +4,4 @@ slow:Ruby String literals with a magic frozen comment produce the same object fo
 slow:Ruby String literals with a magic frozen comment produce different objects for literals with the same content in different files if the other file doesn't have the comment
 slow:Ruby String literals with a magic frozen comment produce different objects for literals with the same content in different files if they have different encodings
 slow:Ruby String literals with a magic frozen comment interpolated string is frozen
+slow:Ruby String literals with a magic frozen comment when disabled interpolated string is not frozen