Add a test for SanitizationFilter::ANCHOR_SCHEMES
Add a test for SanitizationFilter::ANCHOR_SCHEMES

Clients may come to rely on this, so we should have a test to ensure we
don't break them.

jch/html-pipeline,kbrock/html-pipeline,gjtorikian/html-pipeline,jch/html-pipeline,moskvax/html-pipeline,github/html-pipeline,github/html-pipeline,terracatta/html-pipeline,moskvax/html-pipeline,terracatta/html-pipeline,gjtorikian/html-pipeline,kbrock/html-pipeline