gh-136306: fix `test_ssl.ContextTests.test_set_groups` on FIPS builds (#137405)

X25519 is not a valid curve if OpenSSL is built with FIPS mode,
and ignoring unknown groups in `SSL_CTX_set1_groups_list()`
is only supported since OpenSSL 3.3, so we use two curves that
are known to be FIPS-compliant, namely P-256 and P-384.