wgengine/magicsock: fix CI-only test warning of missing health tracker

While looking at deflaking TestTwoDevicePing/ping_1.0.0.2_via_SendPacket,
there were a bunch of distracting:

    WARNING: (non-fatal) nil health.Tracker (being strict in CI): ...

This pacifies those so it's easier to work on actually deflaking the test.

Updates #11762
Updates #11874

Change-Id: I08dcb44511d4996b68d5f1ce5a2619b555a2a773
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/magicsock/magicsock_test.go

@@ -176,6 +176,7 @@ func newMagicStackWithKey(t testing.TB, logf logger.Logf, l nettype.PacketListen
 	if err != nil {
 		t.Fatalf("netmon.New: %v", err)
 	}
+	ht := new(health.Tracker)
 
 	var reg usermetric.Registry
 	epCh := make(chan []tailcfg.Endpoint, 100) // arbitrary
@@ -183,6 +184,7 @@ func newMagicStackWithKey(t testing.TB, logf logger.Logf, l nettype.PacketListen
 		NetMon:                 netMon,
 		Metrics:                &reg,
 		Logf:                   logf,
+		HealthTracker:          ht,
 		DisablePortMapper:      true,
 		TestOnlyPacketListener: l,
 		EndpointsFunc: func(eps []tailcfg.Endpoint) {