cmd/go/internal/work: fix TestRespectGroupSticky on FreeBSD

FreeBSD doesn't allow non-root users to enable the SetGID bit on
files or directories in /tmp, however it does allow this in
subdirectories, so create the test directory one level deeper.

Followup to golang/go#19596.

Change-Id: I30e71c6d6a156badc863e8068df10ef6ed817e26
Reviewed-on: https://go-review.googlesource.com/41216Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/internal/work/build_test.go

@@ -195,8 +195,13 @@ func TestRespectGroupSticky(t *testing.T) {
 	}
 	defer os.RemoveAll(stickydir)
 
-	// Change stickydir's permissions to include group sticky bit.
-	if err := os.Chmod(stickydir, 0755|os.ModeSetgid); err != nil {
+	testdir, err := ioutil.TempDir(stickydir, "testdir")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Change testdir's permissions to include group sticky bit.
+	if err := os.Chmod(testdir, 0755|os.ModeSetgid); err != nil {
 		t.Fatal(err)
 	}
 
@@ -207,7 +212,7 @@ func TestRespectGroupSticky(t *testing.T) {
 	defer os.Remove(pkgfile.Name())
 	defer pkgfile.Close()
 
-	stickyFile := filepath.Join(stickydir, "sticky")
+	stickyFile := filepath.Join(testdir, "sticky")
 	if err := b.moveOrCopyFile(nil, stickyFile, pkgfile.Name(), 0666, true); err != nil {
 		t.Fatalf("moveOrCopyFile: %v", err)
 	}