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 <c22b5f9178342609428d6f51b2c5af4c0bde6a42@filippo.io>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Adam Langley <db68fba232b2c49ed3e9d3081df349dde7b61dfd@golang.org>

cloudflare/go,Cofyc/go,polyverse-security/go,cloudflare/go,CAFxX/go,josharian/go,polyverse-security/go,cloudflare/go,nawawi/go,golang/go,josharian/go,CAFxX/go,cloudflare/go,nawawi/go,akutz/go,cloudflare/go,CAFxX/go,polyverse-security/go,nawawi/go,akutz/go,CAFxX/go,codestation/go,nawawi/go,josharian/go,CAFxX/go,josharian/go,akutz/go,Cofyc/go,akutz/go,nawawi/go,CAFxX/go,polyverse-security/go,akutz/go,golang/go,codestation/go,Cofyc/go,josharian/go,nawawi/go,nawawi/go,codestation/go,codestation/go,codestation/go,christopher-henderson/Go,CAFxX/go,josharian/go,akutz/go,golang/go,codestation/go,cloudflare/go,Cofyc/go,codestation/go,Cofyc/go,CAFxX/go,cloudflare/go,josharian/go,cloudflare/go,golang/go,Cofyc/go,golang/go,golang/go,golang/go,polyverse-security/go,akutz/go,josharian/go,akutz/go,Cofyc/go,polyverse-security/go,polyverse-security/go,nawawi/go,codestation/go,polyverse-security/go,golang/go,Cofyc/go