Tighten the matching when detecting which certificates are in use to
determine algorithm requirements. This fixes a bug whereby all tests were
skipped in configurations without RSA except for an Mbed TLS client against
a GnuTLS or OpenSSL server, due to *server2* matching ssl_server2.
Fixes#8366.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>