Add support for multi-browser testing in Testling

This commit shuffles aspects of testing around in order to allow
[Testling CI][1] to run our browser tests.

[1]: https://ci.testling.com/
Adding deeper requirements for browser testing.
Don't depend on the URL of the test runner
Merge branch 'testling'

I'm not entirely sure this is all working yet, but I can't see any harm
in merging.

Conflicts:
	test/runner.html