WebTransport bfcache tests

[jesup]

Currently the webtransport spec stipulates a bfcache behavior which would lead to a broken state when coming out of bfcache. For this reason, Firefox and apparently Chrome/Edge are blocking documents from bfcache if they have active WebTransports. (Chrome appears to block a few failed/closed WebTransport pages too).

There's an issue ready-for-pr to fix this behavior: w3c/webtransport#326

Firefox has tests that check that blocking is implemented. We could land these as local mozilla-only tests, or we could land them as 'tentative' tests even though this isn't the current or expected bfcache behavior. Which should we do? What do the Chromium people think?

@jgraham @saschanaz @ricea