Enable testing for System.IO.Ports package

[krwq]

Related:

• https://github.com/dotnet/corefx/issues/35358
• https://github.com/dotnet/corefx/issues/33374

When creating a package for native (dotnet/corefx#35126) I have disabled package testing because of some infrastructure issues (build require that a package was already on nuget before it was uploaded) - this should be possible to re-enable now.

[ericstj]

We can do this now by downgrading the native package versions and restoring them from the feed (eg: the last corefx build)

[krwq]

will take a look whenever got time