Check for local installs of Pester first

When testing locally, we should check if the module is there rather than pulling down Pester every time.  I've added a check to see if Pester is found on the system before installing PSGet and before installing Pester.  There is a bit of duplication and it could be made less ugly, but it works now.

I've also added a blank line in between the logging message from Test-Kitchen and the start of the Pester run.