Tests: POP3 commands sent to backend

[VadimZhestikov]

Tests for the POP3 command lines the mail proxy sends to the backend.

Credentials are copied verbatim into USER <login> and PASS <passwd>.
These lines are terminated by CRLF and POP3 has no syntax that can carry a CR
or LF inside a command argument, so such a byte in a credential -- reachable
only with SASL, since plain USER/PASS values come from a parsed command
line -- turns one command into two command lines to the backend.

The test

New mail_proxy_pop3.t covers both the login path (USER) and the password
path (PASS), plus a baseline that a normal AUTH PLAIN login reaches the
backend with no extra command line.

Two notes on how it is built:

• The backend daemon answers exactly one response per command nginx sends and
  records any other line silently, retrievable by the client with XEXTRA
  once logged in, since nginx proxies transparently from that point (the same
  idiom as XPROXY in Test::Nginx::SMTP). Recording rather than answering
  is what keeps the assertions independent of response timing: the shared
  pop3_test_daemon answers -ERR unknown command to the injected line, and
  whether that reply reaches nginx together with the preceding one or on its
  own changes what the client ends up seeing.

• The auth_http stub accepts unconditionally. The precondition for relaying
  such a credential is an auth server that accepts it, and the behaviour under
  test is at the relay layer, independent of auth policy.

The two credential cases are marked TODO, as the fix is not yet released.

Testing

Verified against a patched and an unpatched binary:

• unpatched -- both cases fail, the login succeeds and the CR/LF is relayed;
  the suite stays green through TODO.
• patched -- both cases pass.
• the full mail suite passes on the patched binary (277 tests), including
  mail_pop3.t, mail_imap.t, mail_smtp.t, mail_smtp_xclient.t and
  mail_proxy_protocol.t.

Code PR: nginx/nginx#1620