SQL: Randomize fetch size in tests (elastic/x-pack-elasticsearch#2626)

Now that we have fetch size working consistently we should randomize
the fetch size that we use in the tests to detect any errors caused
by strange fetch sizes.

Original commit: elastic/x-pack-elasticsearch@2c41fb5309

qa/sql/src/main/java/org/elasticsearch/xpack/qa/sql/jdbc/SpecBaseIntegrationTestCase.java

@@ -91,9 +91,7 @@ public abstract class SpecBaseIntegrationTestCase extends JdbcIntegrationTestCas
 
     protected ResultSet executeJdbcQuery(Connection con, String query) throws SQLException {
         Statement statement = con.createStatement();
-        //statement.setFetchSize(randomInt(10));
-        // NOCOMMIT: hook up pagination
-        statement.setFetchSize(1000);
+        statement.setFetchSize(between(1, 500));
         return statement.executeQuery(query);
     }