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

PR-URL: https://github.com/nodejs/node/pull/35883


Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>