ipn/{ipnlocal,localapi}, wgengine/netstack: call (*LocalBackend).Shutdown when tests that create them complete

We have several places where LocalBackend instances are created for testing, but they are rarely shut down
when the tests that created them exit.

In this PR, we update newTestLocalBackend and similar functions to use testing.TB.Cleanup(lb.Shutdown)
to ensure LocalBackend instances are properly shut down during test cleanup.

Updates #12687

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

wgengine/netstack/netstack_test.go

@@ -64,6 +64,7 @@ func TestInjectInboundLeak(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
+	t.Cleanup(lb.Shutdown)
 
 	ns, err := Create(logf, tunWrap, eng, sys.MagicSock.Get(), dialer, sys.DNSManager.Get(), sys.ProxyMapper())
 	if err != nil {
@@ -126,6 +127,7 @@ func makeNetstack(tb testing.TB, config func(*Impl)) *Impl {
 	if err != nil {
 		tb.Fatalf("NewLocalBackend: %v", err)
 	}
+	tb.Cleanup(lb.Shutdown)
 
 	ns.atomicIsLocalIPFunc.Store(func(netip.Addr) bool { return true })
 	if config != nil {