REGTESTS: sample_fetches: test the req.ssl_alpn fetch

REGTESTS: sample_fetches: test the req.ssl_alpn fetch

req.ssl_alpn was the last fetch of the raw ClientHello family without a
test of its own, it only appeared as a condition in the tcp-check test.
It is also the only one of the family which returns several samples,
one per protocol name of the extension, so the test checks that a
log-format only sees the first name while an ACL matches any of them.

Five hand-crafted ClientHellos are used: one advertising "h2" and
"http/1.1" behind a server_name extension, one advertising the single
"acme-tls/1" name of the documentation example, one without any ALPN
extension, one whose ALPN extension is too short to hold a name, and
one announcing a name longer than the extension which carries it, the
last two checking that the fetch gives up instead of reading the
extension which follows.