http2: small fixes to compatibility layer

This is some assorted work on the compatibility layer that doesn't warrant its own PRs:

• Expand argument validation through compat API
• adjust behaviour of response.end to not throw if stream already closed to match http1
• adjust behaviour of response.writeContinue to not throw if stream already closed and other very small tweaks
• Add tests for added and fixed behaviour
• Add tests for edge case behaviours of setTimeout, createPushResponse, destroy, end and trailers

I'll add some additional comments on the code itself, so see below for those.

As always, happy to implement any feedback. Thanks in advance~

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

http2, test