os: skip TestReadlink sub-tests requiring symlinks when not enough

permissions are held

Some of the TestReadlink sub-tests require os.Symlink to succeed.
If the user doesn't have enough permissions to create symlinks, then
there is no point in running the test.

Change-Id: I06ec7e3ddf0016e804667bba0ee6ebe6baa01872
Reviewed-on: https://go-review.googlesource.com/c/go/+/614655
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Auto-Submit: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>

src/os/os_windows_test.go

@@ -1269,6 +1269,9 @@ func TestReadlink(t *testing.T) {
 		}
 
 		t.Run(name, func(t *testing.T) {
+			if !tt.junction {
+				testenv.MustHaveSymlink(t)
+			}
 			if !tt.relative {
 				t.Parallel()
 			}