cmd/go: silence TLS handshake errors during test

cmd/go: silence TLS handshake errors during test

We don't need to see messages like "TLS handshake error: EOF"
during go test.

Change-Id: If6bf51e655119914f337b9e61448c99485af34f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/718183
Reviewed-by: Michael Matloob <matloob@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>