Expand on Extension Tests

Tests to add for DotnetCoreAcquisitionExtension:

- Ensuring Linux dependencies properly detects failure cases. This will be quite a bit harder (stretch goal) because you need to fabricate .NET apps that explode in known ways to indicate missing dependencies.

Edit 12/11/19, 1/15/20: Completed test cases:
- Installing/ uninstalling multiple versions
- Invalid dotnet version fails gracefully with appropriate error
- Bad network (can be mocked with custom dotnet-install scripts) results in appropriate error
- Commands besides install/uninstall
- Check it doesn't re-download the same version twice