mcs: make scheduling server test stable

[lhy1024]

What problem does this PR solve?

Issue Number: Close #xxx

What is changed and how does it work?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has the configuration change
• Has HTTP API interfaces changed (Don't forget to add the declarative for the new API)
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• PR to update pingcap/tiup:
• Need to cherry-pick to the release branch

Release note

None.

[codecov-commenter]

Codecov Report

Merging #26 (c76c4de) into master (86831ce) will increase coverage by 0.01%.
The diff coverage is 66.66%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #26      +/-   ##
==========================================
+ Coverage   74.29%   74.31%   +0.01%     
==========================================
  Files         451      451              
  Lines       48863    48872       +9     
==========================================
+ Hits        36305    36317      +12     
+ Misses       9349     9348       -1     
+ Partials     3209     3207       -2     

Flag	Coverage Δ
unittests	74.31% <66.66%> (+0.01%)	⬆️

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