Test for expiration attribute value

All,

I have proposed a test for the expiration attribute:
https://github.com/w3c/web-platform-tests/pull/3798

To implement this test, I switched to the Upfront Authentication Method
with the DRMToday servers (vs the Test Dummy method we are using today).

You can't have both at once, so we will need to coordinate when to merge
this test with the switchover of the server. All users of the repo will
need to pick up the change at the same time as we update the server (if
they want their tests to keep working).

I will leave it in a PR for review for a while and leave the server set to
Test Dummy.

Note that if you want to try it out by pulling the branch from my fork you
will need to get the server temporarily switched over. LMK if you need to
do this.

Good news is that the test passes on at least Chrome and Firefox.

...Mark