html: convert fuzz test to native Go fuzzing

Convert the existing gofuzz based fuzz test to a testing.F based fuzz
test.

Change-Id: Ieae69ba7fb17bd54d95c7bb2f4ed04c323c9f15f
Reviewed-on: https://go-review.googlesource.com/c/go/+/494195
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>

Cherry-picked-from: golang/go@200a01f

html/fuzz_test.go

@@ -0,0 +1,22 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package html
+
+import "testing"
+
+func FuzzEscapeUnescape(f *testing.F) {
+	f.Fuzz(func(t *testing.T, v string) {
+		e := EscapeString(v)
+		u := UnescapeString(e)
+		if u != v {
+			t.Errorf("EscapeString(%q) = %q, UnescapeString(%q) = %q, want %q", v, e, e, u, v)
+		}
+
+		// As per the documentation, this isn't always equal to v, so it makes
+		// no sense to check for equality. It can still be interesting to find
+		// panics in it though.
+		EscapeString(UnescapeString(v))
+	})
+}