Remove unnecessary clearing of event handlers in resource selection tests

Nothing that happens after t.done() will cause a test to fail, not even
immediately throwing an exception or calling assert_unreached().
Remove unnecessary timeout in candidate remove tests

These tests aren't really special, they can have the default timeout
using the default timeout mechanism.
Merge two almost identical source candidate remove tests

The only difference is onerror vs addEventListener. Keep addEventListener
since onerror is already used a lot in the other tests.
Add "candidate" to a candidate source test title
Remove incorrect comments about when resource selection is run

They used to be true, but inserting a media element into a document
no longer invokes resource selection.