Provide a better comparison utility to check query results

[yibin87]

Current check can't work correctly for order by + limit queries :
MySQL specification doesn't guaratee the order of identical values.
Order by alone will generate sort operator, and will execute in TiDB only.
While order by + limit will generate two level TopN operators, one in coprocessor, one in TiDB, which might introduce implementation diefferences. And they are all right according to MySQL specification. But different orders of identical values will fail our copr test.
https://dev.mysql.com/doc/refman/5.6/en/limit-optimization.html

One current workaround pr, which removed limit operator, to pass the test. #162