REGTEST: add a few HTTP messaging tests

REGTEST: add a few HTTP messaging tests

These tests send GET/HEAD/POST requests in H1 and H2, with and without
HTX, with and without a body, and verify that the behaviour is the expected
one. For now HEAD requests have been commented out because in H1 they are
not really testable as varnishtest expects to read a body, and in H2 the
behaviour depends on HTX/legacy, indicating a bug in haproxy (it looks
like we can deliver some data in response to HEAD in legacy mode).