global sort: test the global sort whole process in ut

## Enhancement
Currently, global sort contains 3 steps:
1. write sorted files to s3.
2. merge sort sorted files and write to s3.
3. read files by key range and sort them.

But the whole process only run in integration test in ci with only 1 tidb node.
We need a way to mimic the dist execution process in UT to make the planning process and execution process more reliable and easier to debug during developing.