crypto/tls: fix and expand TestVerifyPeerCertificate and TestGetClientCertificate

crypto/tls: fix and expand TestVerifyPeerCertificate and TestGetClientCertificate

TestGetClientCertificate had disabled verification, and was only passing
because it was mistakenly checking for empty verifiedChains.

Change-Id: Iea0ddbdbbdf8ac34b499569820a2e4ce543a69c7
Reviewed-on: https://go-review.googlesource.com/47430
Run-TryBot: Filippo Valsorda <hi@filippo.io>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>