Fix flakiness in TestSemiSyncRequiredWithTwoPC

[GuptaManan100]

Description

This PR fixes flakiness in TestSemiSyncRequiredWithTwoPC.

Previously, after the test was finished, we would run PRS in a defer function. But since we were PRSsing to the same tablet, we didn't actually end up changing the semi-sync settings. VTOrc would fix this, but it would race with the next test running.

This PR fixes this problem, by changing the primary first in the test itself that ensures semi-sync changes have taken effect.

Related Issue(s)

• RFC: Atomic Distributed Transactions #16245

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.65%. Comparing base (1cb39b0) to head (d95f5c7).
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17332      +/-   ##
==========================================
+ Coverage   67.41%   67.65%   +0.24%     
==========================================
  Files        1574     1578       +4     
  Lines      253313   255931    +2618     
==========================================
+ Hits       170765   173152    +2387     
- Misses      82548    82779     +231     

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