[SRI] Some new tests

FYI, if anyone wants to review, I've updated (pretty much re-written) the
existing tests for subresource integrity to reflect the current syntax and
feature set, if anyone wants to review or check against their
implementation. (there are still some TODOs, e.g. no testing of <link> yet,
but this is what I'll likely have time to do this week on this)

https://github.com/w3c/web-platform-tests/pull/1820
https://critic.hoppipolla.co.uk/r/4968

-Brad