signatureSchemesForCertificate was written to be used with TLS 1.3, but
ended up used for TLS 1.2 client certificates in a refactor. Since it
only supported TLS 1.3 signature algorithms, it would lead to no RSA
client certificates being sent to servers that didn't support RSA-PSS.

TestHandshakeClientCertRSAPKCS1v15 was testing *specifically* for this,
but alas the OpenSSL flag -verify accepts an empty certificates list as
valid, as opposed to -Verify...

Fixes #28925

Change-Id: I61afc02ca501d3d64ab4ad77bbb4cf10931e6f93
Reviewed-on: https://go-review.googlesource.com/c/151660
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>