Improved asynchronous code in Marten tests

[glen-84]

Summary of the changes (Less than 80 chars)

• Improved asynchronous code in Marten tests.

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I missed this when making these methods async.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.18%. Comparing base (2f07470) to head (9f814e8).
Report is 2 commits behind head on main-version-14.

Additional details and impacted files

@@                 Coverage Diff                 @@
##           main-version-14    #7747      +/-   ##
===================================================
+ Coverage            74.17%   74.18%   +0.01%     
===================================================
  Files                 2859     2859              
  Lines               149427   149427              
  Branches             17348    17348              
===================================================
+ Hits                110837   110852      +15     
+ Misses               32799    32784      -15     
  Partials              5791     5791              

Flag	Coverage Δ
unittests	74.18% <ø> (+0.01%)	⬆️

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