REGTESTS: http-messaging: test the options dropping the HTTP trailers

REGTESTS: http-messaging: test the options dropping the HTTP trailers

"option http-drop-request-trailers" and
"option http-drop-response-trailers" had no coverage.

vtest can send HTTP/1 trailers but cannot parse them, so neither end of
the chain may be a vtest HTTP peer when the trailers are expected to be
forwarded: the request side origin is a small haproxy instance, and the
client of the forwarded response side only reads the headers and then
the exact number of remaining bytes.

What actually leaves the proxy is observed at the byte level by two
extra haproxy instances running in TCP mode, one on each side, which
report through their log whether the trailer field name appears in what
they relayed. This gives a positive assertion for both the dropped and
the forwarded case, and the "no option" form is covered as well.