Multi-session tests

All,

I proposed some changes to the multi-session tests:
https://github.com/w3c/web-platform-tests/pull/3629

Could someone take a look ?

The multi-key test content we presently have uses multiple keys in every
fragment, so we can use this for a multi-key test with multiple sessions
that are created / provided with licenses up front.

I would also like to add a test where one key is used for a few seconds
followed by another key being used, with a separate session for each key
and the second session being created only after playback stops at the end
of the portion encrypted with the first key. But we need different content
for that. I will look into that tomorrow.

...Mark