Fix 80-test_cmp_http.t to be more flexible regarding IP versions

Fix 80-test_cmp_http.t to be more flexible regarding IP versions

Because apps/lib/http_server.c had a hard coded "[::]" for the accept host,
80-test_cmp_http.t assumed that it would always get a CMP server on an IPv6
address, and tested for that.

With the fix in apps/lib/http_server.c, that test was of course doomed to
fail.  Since CMP should be about IP version testing, 80-test_cmp_http.t is
adapted to allow the Mock server to accept connections on either IP version,
and the test for IPv6 is removed.

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/21989)