Add automated tests for kvm.ps1

Added a bunch of automated tests using [Pester](https://github.com/Pester/Pester).

Note: I needed to patch Pester to write marker strings to stdout for TeamCity to read (as per https://confluence.jetbrains.com/display/TCD9/Build+Script+Interaction+with+TeamCity). The patch is in a personal fork of Pester.

It's all integrated into the makefile, so it works on the CI and reports detailed test failure information to the Tests tab for the build on TeamCity.

There are a couple scenarios that are not covered (they involved a lot more machine-level config I didn't want to mess with in automated tests):
* Installation of kvm.ps1 itself
* kvm use -persistent
It could definitely be the dev feed. Basically, all I need is a fairly stable feed that has a known set of KRE packages on it. Since part of the tests work with explicit version numbers (to verify that part) I used beta1 on master because it's there and it's fairly consistent. The KRE used in these tests doesn't really matter since the goal isn't to test those KREs, just to test KVM's ability to retrieve them.