test: Use serial dispatch queue for invocations

[philipphofmann]

Using the barrier flag with the concurrent attribute is similar to using a serial dispatch queue, so we can use a serial dispatch queue instead to simplify the syntax.

#skip-changelog

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 90.859%. Comparing base (1ac58a6) to head (8b0ecd7).

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #3981       +/-   ##
=============================================
- Coverage   90.870%   90.859%   -0.012%     
=============================================
  Files          594       594               
  Lines        46083     46082        -1     
  Branches     16442     16441        -1     
=============================================
- Hits         41876     41870        -6     
- Misses        4027      4142      +115     
+ Partials       180        70      -110     

Files	Coverage Δ
SentryTestUtils/Invocations.swift	87.500% <66.666%> (ø)

... and 30 files with indirect coverage changes

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