fix: delete event cache deadlock test flakiness #15955

[nromriell]

Closes #15955

The timing between the two test goroutines could cause issues trying to re-create the deadlock condition causing the test to occasionally fail. Fixed by adding the same wait for condition that was already present in the EnsureSynced function to the Invalidate function as well as a condition to wait until both functions had completed.

Some minor cleanup of the test and naming to make it slightly more readable

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[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (6dbd47e) 49.67% compared to head (d38bbc4) 49.66%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #15964      +/-   ##
==========================================
- Coverage   49.67%   49.66%   -0.01%     
==========================================
  Files         267      267              
  Lines       46383    46383              
==========================================
- Hits        23039    23036       -3     
- Misses      21084    21086       +2     
- Partials     2260     2261       +1     

see 2 files with indirect coverage changes

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[leoluz]

LGTM
Thank you!