util/testenv: add func to report whether a testing.TB is in parallel mode

For future in-memory network changes (#15558) to be able to be
stricter and do automatic leak detection when it's safe to do so, in
non-parallel tests.

Updates tailscale/corp#27636

Change-Id: I50f03b16a3f92ce61a7ed88264b49d8c6628f638
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/testenv/testenv_test.go

@@ -16,3 +16,16 @@ func TestDeps(t *testing.T) {
 		},
 	}.Check(t)
 }
+
+func TestInParallelTestTrue(t *testing.T) {
+	t.Parallel()
+	if !InParallelTest(t) {
+		t.Fatal("InParallelTest should return true once t.Parallel has been called")
+	}
+}
+
+func TestInParallelTestFalse(t *testing.T) {
+	if InParallelTest(t) {
+		t.Fatal("InParallelTest should return false before t.Parallel has been called")
+	}
+}