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/integration/integration_test.go

@@ -73,9 +73,7 @@ func TestMain(m *testing.M) {
 // https://github.com/tailscale/tailscale/issues/7894
 func TestTUNMode(t *testing.T) {
 	tstest.Shard(t)
-	if os.Getuid() != 0 {
-		t.Skip("skipping when not root")
-	}
+	tstest.RequireRoot(t)
 	tstest.Parallel(t)
 	env := NewTestEnv(t)
 	env.tunMode = true
@@ -1565,9 +1563,7 @@ func testAutoUpdateDefaults(t *testing.T, useCap bool) {
 // https://github.com/tailscale/corp/issues/22511
 func TestDNSOverTCPIntervalResolver(t *testing.T) {
 	tstest.Shard(t)
-	if os.Getuid() != 0 {
-		t.Skip("skipping when not root")
-	}
+	tstest.RequireRoot(t)
 	env := NewTestEnv(t)
 	env.tunMode = true
 	n1 := NewTestNode(t, env)
@@ -1637,9 +1633,7 @@ func TestDNSOverTCPIntervalResolver(t *testing.T) {
 // directions.
 func TestNetstackTCPLoopback(t *testing.T) {
 	tstest.Shard(t)
-	if os.Getuid() != 0 {
-		t.Skip("skipping when not root")
-	}
+	tstest.RequireRoot(t)
 
 	env := NewTestEnv(t)
 	env.tunMode = true
@@ -1779,9 +1773,7 @@ func TestNetstackTCPLoopback(t *testing.T) {
 // directions.
 func TestNetstackUDPLoopback(t *testing.T) {
 	tstest.Shard(t)
-	if os.Getuid() != 0 {
-		t.Skip("skipping when not root")
-	}
+	tstest.RequireRoot(t)
 
 	env := NewTestEnv(t)
 	env.tunMode = true

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) {