Fix NewTestCluster to skip tests when DB is unreachable

pgxpool.NewWithConfig does not connect eagerly, so NewCluster succeeds
even without a running database. Added a Ping check after cluster
creation to reliably skip tests when the database cannot be reached.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

dbxtest/dbxtest.go

@@ -46,6 +46,11 @@ func NewTestCluster(t testing.TB, opts ...dbx.Option) *dbx.Cluster {
 		t.Skipf("dbxtest: cannot connect to test database: %v", err)
 	}
 
+	if err := cluster.Ping(ctx); err != nil {
+		cluster.Close()
+		t.Skipf("dbxtest: cannot reach test database: %v", err)
+	}
+
 	t.Cleanup(func() {
 		cluster.Close()
 	})