Add unified sanitizer smoke test

[schlessera]

Feature description

I think it would be valuable to have a single test that runs a big chunk of HTML through the sanitizer which contains at least 1 validation error for each of the different sanitizer.

This would allow us to ensure that all sanitizers are active and working.

As an example of what this would catch early on, see https://github.com/ampproject/amp-wp/pull/4535#pullrequestreview-389647527, which demonstrates that:

• I'm a fool;
• PRs that have passing tests can still fail at a fundamental level, which would only be caught during QA right now.

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Acceptance criteria

• Have at least 1 test that would fail if any single one of the sanitizers is inactive or ineffective.

Implementation brief

QA testing instructions

Demo

Changelog entry