Remove async void usage

[martincostello]

Remove tests using async void that shouldn't be.

dotnet/extensions#4985 (comment)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.64%. Comparing base (77cfe6d) to head (d1af59b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2013   +/-   ##
=======================================
  Coverage   83.64%   83.64%           
=======================================
  Files         312      312           
  Lines        7105     7105           
  Branches     1054     1054           
=======================================
  Hits         5943     5943           
  Misses        789      789           
  Partials      373      373           

Flag	Coverage Δ
linux	83.64% <ø> (ø)
macos	83.64% <ø> (ø)
windows	?

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