ipn/ipnlocal: rename misnamed DisablePortMapperForTest to DisablePortPollerForTest

I think this was originally a brain-o in 9380e2dfc6. It's
disabling the port _poller_, listing what open ports (i.e. services)
are open, not PMP/PCP/UPnP port mapping.

While there, drop in some more testenv.AssertInTest() in a few places.

Updates #cleanup

Change-Id: Ia6f755ad3544f855883b8a7bdcfc066e8649547b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/lapitest/backend.go

@@ -45,7 +45,7 @@ func newBackend(opts *options) *ipnlocal.LocalBackend {
 		tb.Fatalf("NewLocalBackend: %v", err)
 	}
 	tb.Cleanup(b.Shutdown)
-	b.DisablePortMapperForTest()
+	b.DisablePortPollerForTest()
 	b.SetControlClientGetterForTesting(opts.MakeControlClient)
 	return b
 }