Add test for CVE-2020-1967

Add to test_sslsigalgs a TLSProxy test that injects a
"signature_algorithms_cert" extension that contains an unallocated
codepoint.

The test currently fails, since s_server segfaults instead of
ignoring the unrecognized value.

Since "signature_algorithms" and "signature_algorithms_cert" are very
similar, also add the analogous test for "signature_algorithms".

Reviewed-by: Matt Caswell <matt@openssl.org>
