[master] Update dependencies from dotnet/arcade dotnet/core-setup dotnet/corefx ...

[master] Update dependencies from dotnet/arcade dotnet/core-setup dotnet/corefx (mono/mono#17458)

* Update dependencies from https://github.com/dotnet/arcade build 20191018.2

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.19518.2
- Microsoft.DotNet.Helix.Sdk - 5.0.0-beta.19518.2

* Update dependencies from https://github.com/dotnet/core-setup build 20191021.2

- Microsoft.NETCore.App - 5.0.0-alpha1.19521.2

* Update dependencies from https://github.com/dotnet/corefx build 20191020.7

- Microsoft.Private.CoreFx.NETCoreApp - 5.0.0-alpha1.19520.7

* Update dependencies from https://github.com/dotnet/arcade build 20191023.3

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.19523.3
- Microsoft.DotNet.Helix.Sdk - 5.0.0-beta.19523.3

* Update dependencies from https://github.com/dotnet/core-setup build 20191023.10

- Microsoft.NETCore.App - 5.0.0-alpha1.19523.10

* Update dependencies from https://github.com/dotnet/corefx build 20191023.8

- Microsoft.Private.CoreFx.NETCoreApp - 5.0.0-alpha1.19523.8

* Disable a file system watcher test on mac

See https://github.com/mono/mono/issues/17547

* Reenable tests that were fixed

Commit migrated from https://github.com/mono/mono/commit/a7287bdae54c0539a74fdbe87eb73ba268a95ed6