tstest: add RequireRoot helper

Start using a common helper for tests to declare that they require root.

This is step 1. A later step will then make this helper track which tests were
skipped so a subsequent pass will run these test as root.

Updates tailscale/corp#40007

Change-Id: I4979e1def0fa3691d38c83f48c89aaa443e7f62e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/router/osrouter/router_linux_test.go

@@ -1157,9 +1157,7 @@ func (lt *linuxTest) Close() error {
 }
 
 func newLinuxRootTest(t *testing.T) (*linuxTest, *eventbus.Bus) {
-	if os.Getuid() != 0 {
-		t.Skip("test requires root")
-	}
+	tstest.RequireRoot(t)
 
 	lt := new(linuxTest)
 	lt.tun = createTestTUN(t)