Add abiltity to skip tests in testrtc by using a "?test_filter=TestA,TestB".

Tested: locally with "python -m SimpleHttpServer" with no parameters and
with parameters "?test_filter=MicrophoneTest".
Addressed stefanholmer comments.
Fixed newlines between functions.
Merge pull request #132 from GoogleChrome/add-test-filters

Add abilitity to skip tests in testrtc by using a "?test_filter=TestA,TestB"