Fix flaky test TestShardSync

[GuptaManan100]

Description

This PR fixes the flakiness in the test TestShardSync.
It was noticed that the test was flaky. The problem was that when the tablet is promoted to a primary type, it starts shard sync, but it happens asynchronously. In the meantime we also update the shard record in the topo server. If the shard record is updated before the shard watch for the record has started, the tablet doesn't run a shard sync loop for 30 seconds, and that fails the test.

This PR fixes the test by adding an explicit wait for the shard watch to have started.

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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[vitess-bot]

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[mattlord]

Thanks, @GuptaManan100 ! ❤️

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.16%. Comparing base (4e385ce) to head (f56073c).
Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #17095    +/-   ##
========================================
  Coverage   67.16%   67.16%            
========================================
  Files        1571     1571            
  Lines      252097   252249   +152     
========================================
+ Hits       169318   169424   +106     
- Misses      82779    82825    +46     

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