Create integration tests for the command line interface

[cr7pt0gr4ph7]

Following some bugs with the command line interface, namely:

• Issue with packet pack and multiple paket.template files #893 (Issues with paket pack)
• find-packages not working in 1.16.0 #889 (paket find-packages not working in 1.16.0)
• sorting find-package-versions #886 (paket find-package-versions sorts the output in the wrong order)

I think that it might make sense to create integration tests for the command line interface, using a known input state (i.e. local package repository, known paket.template setup) and comparing it to a known output state.

See, for example, the EndToEnd tests of the Official NuGet client (this is a GitHub mirror of the Codeplex repository), which use a test framework written in PowerShell, and could serve as an orientation.