os: don't silently skip test

This is a followup to CL 3676.

Rather than silently returning from the test, a pass,
use the Skip facility to mark the test as skipped.

Change-Id: I90d237e770150bf8d69f14fb09874e70894a7f86
Reviewed-on: https://go-review.googlesource.com/3682Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/path_test.go

@@ -207,12 +207,13 @@ func TestMkdirAllAtSlash(t *testing.T) {
 		t.Skipf("skipping on %s", runtime.GOOS)
 	}
 	RemoveAll("/_go_os_test")
-	err := MkdirAll("/_go_os_test/dir", 0777)
+	const dir = "/go_os_test/dir"
+	err := MkdirAll(dir, 0777)
 	if err != nil {
 		pathErr, ok := err.(*PathError)
 		// common for users not to be able to write to /
 		if ok && (pathErr.Err == syscall.EACCES || pathErr.Err == syscall.EROFS) {
-			return
+			t.Skipf("could not create %v: %v", dir, err)
 		}
 		t.Fatalf(`MkdirAll "/_go_os_test/dir": %v`, err)
 	}