Fixes #3930 - Splits tests to Tests/UnitTests, Tests/IntegrationTests, Tests/StressTests

[tig]

Fixes

• Fixes Tests are crashing again... #3930
• Fixes Enable unit tests to run in parallel #3007

Proposed Changes/Todos

• Remove dead tests
• Split tests to Tests/UnitTests, Tests/IntegrationTests, Tests/StressTests
• Add Directory.Build.props to unify settings
• Add Direcotry.Packages.props to unify package versions
• Add a script for repeatedly running tests on Ubuntu locally to see if the failure can be reproduced
• Create new ./Tests/UnitTestsParallelizable project and move obviously parallelizable tests.
• Keep trying shit until tests work reliably

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working

[tig]

Another result (after tyring to fix stuff):

  Passed Terminal.Gui.ViewTests.HasFocusChangeEventTests.HasFocusChanged_With_SubView_Raises [< 1 ms]
  Passed Terminal.Gui.ViewTests.HasFocusChangeEventTests.HasFocusChanging_RemoveFocus_Raises [< 1 ms]
Error opening terminal: unknown.
The active test run was aborted. Reason: Test host process crashed : Error opening terminal: unknown.

Data collector 'Blame' message: System.IO.FileNotFoundException: Collect dump was enabled but no dump file was generated.
   at Microsoft.TestPlatform.Extensions.BlameDataCollector.ProcessDumpUtility.GetDumpFiles(Boolean warnOnNoDumpFiles, Boolean processCrashed) in /_/src/Microsoft.TestPlatform.Extensions.BlameDataCollector/ProcessDumpUtility.cs:line 84
   at Microsoft.TestPlatform.Extensions.BlameDataCollector.BlameCollector.SessionEndedHandler(Object sender, SessionEndEventArgs args) in /_/src/Microsoft.TestPlatform.Extensions.BlameDataCollector/BlameCollector.cs:line 534.

Test Run Aborted.
Total tests: Unknown
     Passed: 791
    Skipped: 10
 Total time: 41.9252 Seconds

The active Test Run was aborted because the host process exited unexpectedly. Please inspect the call stack above, if available, to get more information about where the exception originated from.
The test running when the crash occurred: 
Terminal.Gui.ApplicationTests.ApplicationScreenTests.ClearScreenNextIteration_Resets_To_False_After_LayoutAndDraw

[BDisp]

@tig I sent a PR to your branch (tig#44) for leverage using link files for the TestHelpers file instead of using copies.

[tig]

Perhaps this refactoring has hidden the root bug that was causing the crash, but I've not gotten a failure in 3 runs.

Please review so I can merge and we can leverage all of this. THanks.