crypto/tls: enable signature algorithm BoGo tests (and fix two bugs)

crypto/tls: enable signature algorithm BoGo tests (and fix two bugs)

The two bugs are very minor:

- We were trying to set the ConnectionState CurveID field even if the
  RSA key exchange was in use

- We were sending the wrong alert from TLS 1.2 clients if none of the
  certificate signature algorithms were supported

Change-Id: I6a6a46564f5a9f1a5d44e54fc59a650118ad67d5
Reviewed-on: https://go-review.googlesource.com/c/go/+/675918
Auto-Submit: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Michael Knyszek <mknyszek@google.com>