Add tests for DiagnosticVerifier

[sharwell]

This pull request is a substantial improvement to tests covering the base DiagnosticVerifier.

[codecov]

Codecov Report

Merging #2425 into master will increase coverage by 0.1%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #2425     +/-   ##
=========================================
+ Coverage   96.45%   96.55%   +0.1%     
=========================================
  Files         716      717      +1     
  Lines       92577    92927    +350     
  Branches     3444     3445      +1     
=========================================
+ Hits        89296    89728    +432     
+ Misses       2389     2316     -73     
+ Partials      892      883      -9

Impacted Files	Coverage Δ
...tyleCop.Analyzers.Test/Helpers/DiagnosticResult.cs	97.02% <100%> (+12.65%)	⬆️
...nalyzers.Test/Verifiers/DiagnosticVerifierTests.cs	100% <100%> (ø)
...Cop.Analyzers.Test/Verifiers/DiagnosticVerifier.cs	90.95% <0%> (+24.43%)	⬆️
...eCop.Analyzers/Helpers/ObjectPools/ObjectPool`1.cs	87.03% <0%> (+29.62%)	⬆️

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