Remove exclusion for System.Threading.Tasks.Tests (#23818)
* Update exclusion for System.Threading.Tasks.Tests
There is a race in the test, which causes failures in various stress runs. And that is not Linux specific.
The failure is addressed in https://github.com/dotnet/corefx/pull/36645
This is to update the exclusion lists to make it not just for Linux and to link to the fix PR, so that it could be tracked when it makes it into coreclr.
* Update corefx_test_exclusions.txt
* Update corefx_test_exclusions.txt