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.
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.
Merge pull request #683 from foolip/resource-selection-nitpick

Nitpick the media resource selection tests