test: unflake ITDmlReturningTest (googleapis#3695)

Fixes googleapis#3575
Fixes googleapis#3342
Fixes googleapis#3274
Fixes googleapis#3174
Fixes googleapis#3169
Fixes googleapis#3059
Fixes googleapis#3002

Author: olavloite

google-cloud-spanner/src/test/java/com/google/cloud/spanner/it/ITDmlReturningTest.java

@@ -272,6 +272,8 @@ private List<Struct> executeQuery(long expectedCount, String stmt) {
     List<Struct> rows = new ArrayList<>();
     final TransactionCallable<Void> callable =
         transaction -> {
+          // Make sure we start with an empty list if the transaction is aborted and retried.
+          rows.clear();
           ResultSet resultSet = transaction.executeQuery(Statement.of(stmt));
           // resultSet.next() returns false, when no more row exists.
           // So, number of times resultSet.next() returns true, is the number of rows
@@ -335,6 +337,7 @@ private List<Struct> executeQueryAsync(long expectedCount, String stmt) {
     List<Struct> rows = new ArrayList<>();
     final TransactionCallable<Void> callable =
         transaction -> {
+          rows.clear();
           AsyncResultSet rs = transaction.executeQueryAsync(Statement.of(stmt));
           rs.setCallback(
               Executors.newSingleThreadExecutor(),