test(firestore): del skip in TestTransactionErrors (#2650)

Closes #1708
diff --git a/firestore/transaction_test.go b/firestore/transaction_test.go
index 2f728f5..60d1af6 100644
--- a/firestore/transaction_test.go
+++ b/firestore/transaction_test.go
@@ -150,7 +150,6 @@
 }
 
 func TestTransactionErrors(t *testing.T) {
-	t.Skip("https://github.com/googleapis/google-cloud-go/issues/1708")
 	ctx := context.Background()
 	const db = "projects/projectID/databases/(default)"
 	c, srv, cleanup := newMock(t)