cmd/go: include -errorsas in go test vet checks

Make "go test" run the new errorsas vet check by default.

Fixes #31213.

Change-Id: I5c93c000874ffe1c0b6d647bf10de803f414c5c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/179977
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>

src/cmd/go/internal/test/test.go

@@ -507,6 +507,7 @@ var testVetFlags = []string{
 	// "-cgocall",
 	// "-composites",
 	// "-copylocks",
+	"-errorsas",
 	// "-httpresponse",
 	// "-lostcancel",
 	// "-methods",