all: close fake userspace engines when tests complete

We were leaking FDs.
In a few places, switch from defer to t.Cleanup.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

ipn/ipnlocal/local_test.go

@@ -443,6 +443,7 @@ func TestLazyMachineKeyGeneration(t *testing.T) {
 	if err != nil {
 		t.Fatalf("NewFakeUserspaceEngine: %v", err)
 	}
+	t.Cleanup(eng.Close)
 	lb, err := NewLocalBackend(logf, "logid", store, eng)
 	if err != nil {
 		t.Fatalf("NewLocalBackend: %v", err)