test: mark test-http2-respond-file-error-pipe-offset flaky

This is now failing inconsistently across many platforms. This appears to be the result of the addition of mustSucceed being added to the test during testing refactoring.

We should mark flaky until we have figured out what the issue is.

Refs: https://github.com/nodejs/node/issues/35881

/cc @trott @tniessen