wgengine/magicsock: disable the new ping test.

It's extremely flaky in several dimensions, as well as very slow.
It's making the CI completely red all the time without telling us
useful information.

Set RUN_CURSED_TESTS=1 to run locally.

wgengine/magicsock/magicsock_test.go

@@ -13,6 +13,7 @@ import (
 	"net"
 	"net/http"
 	"net/http/httptest"
+	"os"
 	"strings"
 	"sync"
 	"testing"
@@ -286,6 +287,10 @@ func stashDerpers() (cleanupFn func()) {
 }
 
 func TestTwoDevicePing(t *testing.T) {
+	if os.Getenv("RUN_CURSED_TESTS") == "" {
+		t.Skip("test is very broken, don't run in CI until it's reliable.")
+	}
+
 	// Wipe default DERP list, add local server.
 	// (Do it now, or derpHost will try to connect to derp1.tailscale.com.)
 	derpServer, derpAddr, derpCleanupFn := runDERP(t)