tests: benchmark should start the transaction (#150)

The steadyIncrease benchmark only checked out a read-only session from
the pool, but did not actually start the transaction. That is only done
once a statement is issued for the transaction.

Note that issuing the statement does not actually execute the
statement. That is deferred to the first call to ResultSet#next().

google-cloud-spanner/src/test/java/com/google/cloud/spanner/SessionPoolBenchmark.java

@@ -379,7 +379,9 @@ public void steadyIncrease(MockServer server) throws Exception {
     // Checkout maxSessions sessions by starting maxSessions read-only transactions sequentially.
     List<ReadOnlyTransaction> transactions = new ArrayList<>(server.maxSessions);
     for (int i = 0; i < server.maxSessions; i++) {
-      transactions.add(client.readOnlyTransaction());
+      ReadOnlyTransaction tx = client.readOnlyTransaction();
+      tx.executeQuery(MockServer.SELECT1);
+      transactions.add(tx);
     }
     for (ReadOnlyTransaction tx : transactions) {
       tx.close();