CoreFX CI Unix (#18753)

CoreFX CI Unix (#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