Add Parallel Identical Query Tests.

[JimBobSquarePants]

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• I have written a descriptive pull-request title
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• I have provided test coverage for my change (where applicable)

Description

Adding tests to prove AsyncKeyLock works and that #118 was due to a testing issue.

[codecov]

Codecov Report

Merging #119 into master will increase coverage by 1.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #119      +/-   ##
==========================================
+ Coverage   86.89%   87.98%   +1.09%     
==========================================
  Files          44       44              
  Lines        1190     1190              
  Branches      164      164              
==========================================
+ Hits         1034     1047      +13     
+ Misses        106       97       -9     
+ Partials       50       46       -4     

Flag	Coverage Δ
#unittests	87.98% <ø> (+1.09%)	⬆️

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Impacted Files	Coverage Δ
src/ImageSharp.Web/FormatUtilities.cs	100.00% <ø> (ø)
src/ImageSharp.Web/Caching/Doorman.cs	100.00% <0.00%> (+20.63%)	⬆️

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