Adds a sample Fuzz test function to package html based on
https://github.com/dvyukov/go-fuzz-corpus/blob/master/stdhtml/main.go

Updates #19109
Updates #31309

Change-Id: I8c49fff8f70fc8a8813daf1abf0044752003adbb
Reviewed-on: https://go-review.googlesource.com/c/go/+/174301Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>