Add html5lib-based Sanitizer testcases.
Adds simple Sanitizer API testcases, plus a support library to write more
complex ones based on the html5lib testcase format. This is preperatory work
for an overhaul of the Sanitizer API + test suite implementation.
Bug: 356601280
Change-Id: Icbaacf9d73b953c9398abdddea7d9e3a08bb69d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5920436
Reviewed-by: Joey Arhar jarhar@chromium.org
Reviewed-by: Yifan Luo lyf@chromium.org
Commit-Queue: Daniel Vogelheim vogelheim@chromium.org
Cr-Commit-Position: refs/heads/main@{#1369950}