Signing: for Linux and Mac, implement timestamp integrity verification and enable the tests

[heng-liu]

Timestamp integrity verification is not supported in .Net standard 2.0, therefore tests were disabled for Linux and Mac.
As .Net standard 2.1 starts to support Timestamp integrity verification, we need to implement timestamp integrity verification and enable the tests for Linux and Mac.

This is one of the breakdown tasks from NuGet/NuGet.Client#2706

[heng-liu]

The APIs are in netcoreapp2.1 : https://github.com/dotnet/corefx/issues/34687
But not yet in netstandard2.1 : https://github.com/dotnet/corefx/issues/36062

[heng-liu]

Closed as fixed.