[MSE] Test status update

Hi,

I just wanted to give folks a status update. I've commented on all the
outstanding pull requests. It looks like most of the test cases were
already covered by the Blink tests that I submitted or will be covered when my
new pull request <https://github.com/w3c/web-platform-tests/pull/1238> lands.
This new pull request just updates the W3C tests to include the extra test
cases that were created in Blink after I took the snapshot for my initial
MediaSource test submission
<https://github.com/w3c/web-platform-tests/pull/725>. I'd appreciate it if
at least one of the other UA implementors could take a look at these new
tests and provide some feedback.

Thanks,
Aaron