CoreFX CI Unix (dotnet/coreclr#18753)
* Full Unix test support
* Remove Test helper build from build-test
Add msg prefix
Change if condition
* Change Linux URL
* Syntax error
* Fix Merge Conflict
* Address PR Feedback
* Remove duplicate generate_testhost call
* Address PR comments
* Clean up exclusion list
* Disable Client_ReadWriteCancelledToken_Throws_OperationCanceledException
* Disable all flavors of CancelledToken_Throws_OperationCanceledException
* Disable X509StoreTests.Constructor_DefaultStoreName - flaky on OSX
* Update documentation with Linux instructions and workflow
* Disable EventWaitHandleTests.Ctor_InvalidMode - failing on OSX
* Disable Server_ReadWriteCancelledToken_Throws_OperationCanceledException
Commit migrated from https://github.com/dotnet/coreclr/commit/
e3ccef1c21978fa682bc76fd7626dd7ae9871cc0