test: block init flush tests (use new net6 API)

[bruno-garcia]

Seems the InitCacheFlushTimeout feature doesn't have any tests.
We can improve the code on net6.0 but it requires some test first.

See also #1164

[codecov-commenter]

Codecov Report

Merging #1228 (a5a952a) into explore/net6 (92b7100) will increase coverage by 0.08%.
The diff coverage is 100.00%.

❗ Current head a5a952a differs from pull request most recent head 33225f9. Consider uploading reports for the commit 33225f9 to get more accurate results

@@               Coverage Diff                @@
##           explore/net6    #1228      +/-   ##
================================================
+ Coverage         80.59%   80.67%   +0.08%     
================================================
  Files               212      212              
  Lines              6904     6918      +14     
  Branches           1587     1587              
================================================
+ Hits               5564     5581      +17     
+ Misses              809      806       -3     
  Partials            531      531              

Impacted Files	Coverage Δ
...grations/AppDomainUnhandledExceptionIntegration.cs	86.66% <ø> (ø)
...grations/TaskUnobservedTaskExceptionIntegration.cs	91.66% <ø> (ø)
src/Sentry/Internal/AppDomainAdapter.cs	66.66% <ø> (ø)
src/Sentry/Internal/Polyfills.cs	75.00% <ø> (ø)
src/Sentry/Internal/SynchronizedRandom.cs	100.00% <100.00%> (ø)
...ntry/PlatformAbstractions/FrameworkInstallation.cs	62.50% <0.00%> (ø)
src/Sentry/Internal/BackgroundWorker.cs	84.05% <0.00%> (+0.72%)	⬆️
src/Sentry/Infrastructure/TraceDiagnosticLogger.cs	27.27% <0.00%> (+18.18%)	⬆️

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[github-actions]

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