Tidy up tidy usage

Tidy up tidy usage

 * There's a branch path in the sanitizer that depends on $wgUseTidy,
   which means the test output differs from on wiki.

 * In general, we should set these variables to match the wiki behaviour
   in tests.

 * Exposes T92892, Sanitizer removes empty tags when tidy is disabled.

 * Tweaked tests for T19663 to use an extension tag to show that
   HTML5 tags with non-word characters make it through the parser
   intact (before being ultimately sanitized).

Change-Id: I09c72fd739e11a8b757f37dc4c790758d782ad73