Extend SSL_get_negotiated_group() tests for TLS 1.2

Extend SSL_get_negotiated_group() tests for TLS 1.2

We don't implement RFC 7919 named groups for TLS 1.2, so we can
only test the ECDHE case for non-TLS-1.3.

Interestingly, though the test_key_exchange() routine claimed to
be exercising ffdhe2048 with TLS 1.2, the configured ciphers were
incompatible with DHE key exchange, so we ended up just using RSA
key transport and not doing an ephemeral key exchange at all.
Reconfigure the tests to actually exercise ephemeral key exchange
for both the EC and FF cases (even though we don't use the named
group information for the finite-field case).

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14750)