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.
Check that all property descriptors for attributes have a set property.
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
Merge pull request #697 from Ms2ger/window-properties-propdesc-set

Check that all property descriptors for attributes have a set property; r=zcorpan
Add "conformance-checkers" to manifest blacklist.
Merge pull request #698 from w3c/submission/sideshowbarker/manifest-blacklist

Add "conformance-checkers" to manifest blacklist.
adding ability to configure alternate browsers

to be able to run against selendroid / chrome after starting selendroid-standaone-server or chromedriver respectively.