http2: upgrade tests and add fix for non-existing stream

- check in h2 filter recv that stream actually exists
and return error if not
- add test for parallel, extreme h2 upgrades

http2: upgrade tests and add fix for non-existing stream

- check in h2 filter recv that stream actually exists
and return error if not
- add test for parallel, extreme h2 upgrades that fail if
connections get reused before fully switched
- add h2 upgrade upload test just for completeness

Closes #11563

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