test(spanner): check for Aborted error (#3039)

The test did not check whether the query returned an Aborted error. If the transaction
was aborted by Spanner, the Aborted error should be propagated to the transaction runner,
which will automatically retry the transaction.

Fixes #3029
diff --git a/spanner/integration_test.go b/spanner/integration_test.go
index f3a32c2..dd0a09c 100644
--- a/spanner/integration_test.go
+++ b/spanner/integration_test.go
@@ -2314,7 +2314,11 @@
 			SQL: `INSERT INTO Singers (SingerId, FirstName, LastName) VALUES (1, "Umm", "Kulthum")`,
 		})
 		defer iter.Stop()
-		if row, err := iter.Next(); err != iterator.Done {
+		row, err := iter.Next()
+		if ErrCode(err) == codes.Aborted {
+			return err
+		}
+		if err != iterator.Done {
 			t.Fatalf("got results from iterator, want none: %#v, err = %v\n", row, err)
 		}
 		if iter.RowCount != 1 {