cmd/go: fix at least some instances of double compilation

When the packages the tested package depends on don't build,
we weren't getting out early. Added a simple check for a successful
build to an existing early out.

There may be other ways that double compilation arises, but
this fixes the one listed in the issue.
Fixes #5679

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/13036043

src/cmd/go/test.go

@@ -378,7 +378,7 @@ func runTest(cmd *Command, args []string) {
 			a.deps = append(a.deps, b.action(modeInstall, modeInstall, p))
 		}
 		b.do(a)
-		if !testC {
+		if !testC || a.failed {
 			return
 		}
 		b.init()