Add extensive test coverage for SSL_get_negotiated_group()

This is nearly comprehensive, but we cannot exercise the functionality
for PSK-only TLS 1.3 resumption, since openssl talking to openssl will
always negotiate psk_dhe_ke.

Exercise both the TLS 1.3 and 1.2 cases, for initial handshakes
and resumptions, and for ECDHE and FFDHE.
Since RFC 7919 named groups (for FFDHE) are only supported for TLS 1.3,
the TLS 1.2 versions of those scenarios expect to get NID_undef since
the key exchange was not performed using a named group.

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

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