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>

tstest/tstest.go

@@ -95,6 +95,14 @@ func Parallel(t *testing.T) {
 	}
 }
 
+// RequireRoot skips the test if the current user is not root.
+func RequireRoot(tb testing.TB) {
+	tb.Helper()
+	if os.Getuid() != 0 {
+		tb.Skip("skipping test; requires root")
+	}
+}
+
 // SkipOnKernelVersions skips the test if the current
 // kernel version is in the specified list.
 func SkipOnKernelVersions(t testing.TB, issue string, versions ...string) {