check task doesn't check downstream database connection.

[GMHDBJD]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do? If possible, provide a recipe for reproducing the error.
   wrong target-database config in task config

2. What did you expect to see?
   check-task should return error

3. What did you see instead?
   check pass

4. Versions of the cluster

   • DM version (run dmctl -V or dm-worker -V or dm-master -V):

     master
     

We should ping db in check task.