test/recipes/80-test_cmp_http.t: Don't trust $server_port in start_mock_server()

test/recipes/80-test_cmp_http.t: Don't trust $server_port in start_mock_server()

Even if $server_port isn't touched, it's still a number coming from
configuration.  It's therefore not trustable as an indicator that the
ACCEPT line delivered a port number or an error indication.

$accept_msg is used instead to capture the port if there is one, and
be a better indicator of error.

Fixes #15557
Fixes #15571

Reviewed-by: David von Oheimb <david.von.oheimb@siemens.com>
(Merged from https://github.com/openssl/openssl/pull/15580)