ipn/ipnlocal: use tsd.NewSystem instead of &tsd.System in a few more tests

These were likely added after everything else was updated to use tsd.NewSystem,
in a feature branch, and before it was merged back into main.

Updates #15160

Signed-off-by: Nick Khyl <nickk@tailscale.com>

ipn/ipnlocal/state_test.go

@@ -1397,7 +1397,7 @@ func newLocalBackendWithMockEngineAndControl(t *testing.T, enableLogging bool) (
 	dialer := &tsdial.Dialer{Logf: logf}
 	dialer.SetNetMon(netmon.NewStatic())
 
-	sys := &tsd.System{}
+	sys := tsd.NewSystem()
 	sys.Set(dialer)
 	sys.Set(dialer.NetMon())