test: add integration tests in client for DML Returning (#1910)

* test: add integration tests for DML Returning.

Co-authored-by: Knut Olav Løite <koloite@gmail.com>
Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

google-cloud-spanner/src/main/java/com/google/cloud/spanner/ReadContext.java

@@ -160,7 +160,8 @@ ApiFuture<Struct> readRowUsingIndexAsync(
       String table, String index, Key key, Iterable<String> columns);
 
   /**
-   * Executes a query against the database.
+   * Executes a query against the database. Can also execute a DML statement with returning clause
+   * in a read/write transaction.
    *
    * <p>Implementations may or may not block in the initial {@code executeQuery(...)} call; for
    * those that do not, the remote call will be initiated immediately but blocking on the response

google-cloud-spanner/src/main/java/com/google/cloud/spanner/TransactionContext.java

@@ -110,9 +110,10 @@ default ApiFuture<Void> bufferAsync(Iterable<Mutation> mutations) {
   }
 
   /**
-   * Executes the DML statement(s) and returns the number of rows modified. For non-DML statements,
-   * it will result in an {@code IllegalArgumentException}. The effects of the DML statement will be
-   * visible to subsequent operations in the transaction.
+   * Executes the DML statement (which can be a simple DML statement or DML statement with a
+   * returning clause) and returns the number of rows modified. For non-DML statements, it will
+   * result in an {@code IllegalArgumentException}. The effects of the DML statement will be visible
+   * to subsequent operations in the transaction.
    */
   long executeUpdate(Statement statement, UpdateOption... options);
 
@@ -141,10 +142,11 @@ default ResultSetStats analyzeUpdate(
   }
 
   /**
-   * Executes a list of DML statements in a single request. The statements will be executed in order
-   * and the semantics is the same as if each statement is executed by {@code executeUpdate} in a
-   * loop. This method returns an array of long integers, each representing the number of rows
-   * modified by each statement.
+   * Executes a list of DML statements (which can include simple DML statements or DML statements
+   * with returning clause) in a single request. The statements will be executed in order and the
+   * semantics is the same as if each statement is executed by {@code executeUpdate} in a loop. This
+   * method returns an array of long integers, each representing the number of rows modified by each
+   * statement.
    *
    * <p>If an individual statement fails, execution stops and a {@code SpannerBatchUpdateException}
    * is returned, which includes the error and the number of rows affected by the statements that