REGTESTS: http-messaging: test the http-hardening checks on HTTP/2 headers

REGTESTS: http-messaging: test the http-hardening checks on HTTP/2 headers

This is an LLM-generated reg-test covering HTTP/2 hardening checks.

This is the HTTP/2 counterpart of http_hardening_h1.vtc for the checks that
apply to HTTP/2 (folding and Transfer-Encoding cannot appear there). A
rejected request must cause a stream error, except for an invalid method
which results in a 400 like other method errors, and a rejected response a
502. It also checks that a Host header accompanying an identical :authority
is not considered as a duplicate Host header.