verify sanitizated output for regression tests

[willkg]

In pull #241, I added regression tests for bleach.clean. We should verify the output for those regression tests.

[willkg]

Tagging @g-k with this one.

[g-k]

👍 I'm planning to pull in the test page from https://cure53.de/purify and modify it to POST to a test server that calls bleach.clean and plugs the result back into the DOM.

Hopefully we can reuse their karma test runner to easily check against browsers too: https://github.com/cure53/DOMPurify/blob/master/test/karma.conf.js

[willkg]

The output is probably fine.