Additional EME tests

All,

I have uploaded some additional EME test cases here:
https://github.com/mwatson2/web-platform-tests/tree/clearkey-success/encrypted-media

I have not created a pull request, because there is overlap with the Blink
tests.

I have taken a slightly different approach, which is to define one
function, eme_success, which can execute a variety of different test cases
based on a config object passed in. There are currently only four:
temporary / persistent-usage-record with different ordering of setMediaKeys
and setting video.src, but it is easy to add more with different initData
approaches, different media formats and different keysystems.

What approach do we want to take ? The Blink approach of a different file
for every individual case will bloat as we add different session types,
initData types, media formats and keysystems.

On the other hand, each of the Blink test cases is very straightforward to
follow, whereas the combined one is less so.

My branch also includes some mp4 test content, the key for which is in the
clearkeysuccess.html file.

...Mark