REGTESTS: http-messaging: test the http-hardening checks on trailers

REGTESTS: http-messaging: test the http-hardening checks on trailers

This is an LLM-generated reg-test covering hardening checks on trailers.
These are tricky because we don't have anything beyond TCP mode with
content inspection to check them.

Each trailer-related check gets its own profile, applied to a bind line for
request trailers and to a server line for response trailers, with both
HTTP/1 and HTTP/2 on the receiving side. A profile enabling header checks
verifies that these ones do not affect trailers.

vtest cannot parse HTTP/1 trailers, so the request trailers are forwarded in
HTTP/1 to a TCP frontend of the same instance, which waits for the whole
chunked message, then switches to HTTP and replies with the list of fields
it received. Response trailers are delivered to HTTP/2 clients, and the
response body is waited for so that a rejected trailer results in a 502.

Covered: underscore in trailer name (reject and delete), unregistered
trailer fields, deletion of all trailers, folding, CTL and DEL in values.