runtime/race: install alternate packages to temp dir

The content-based staleness code means that

	go run -gcflags=-l helloworld.go

recompiles all of helloworld.go's dependencies with -gcflags=-l,
whereas before it would have assumed installed packages were
up-to-date. In this test, that means every race iteration rebuilds
the runtime and maybe a few other packages. Instead, install them
to a temporary location for reuse.

This speeds the test from 17s to 9s on my MacBook Pro.

Change-Id: Ied136ce72650261083bb19cc7dee38dac0ad05ca
Reviewed-on: https://go-review.googlesource.com/73992Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/race/output_test.go

@@ -19,6 +19,16 @@ import (
 )
 
 func TestOutput(t *testing.T) {
+	pkgdir, err := ioutil.TempDir("", "go-build-race-output")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(pkgdir)
+	out, err := exec.Command(testenv.GoToolPath(t), "install", "-race", "-pkgdir="+pkgdir, "-gcflags=-l", "testing").CombinedOutput()
+	if err != nil {
+		t.Fatalf("go install -race: %v\n%s", err, out)
+	}
+
 	for _, test := range tests {
 		if test.goos != "" && test.goos != runtime.GOOS {
 			t.Logf("test %v runs only on %v, skipping: ", test.name, test.goos)
@@ -47,7 +57,7 @@ func TestOutput(t *testing.T) {
 			t.Fatalf("failed to close file: %v", err)
 		}
 		// Pass -l to the compiler to test stack traces.
-		cmd := exec.Command(testenv.GoToolPath(t), test.run, "-race", "-gcflags=-l", src)
+		cmd := exec.Command(testenv.GoToolPath(t), test.run, "-race", "-pkgdir="+pkgdir, "-gcflags=-l", src)
 		// GODEBUG spoils program output, GOMAXPROCS makes it flaky.
 		for _, env := range os.Environ() {
 			if strings.HasPrefix(env, "GODEBUG=") ||