Unpack scripts under test automatically

[bcullman]

Background
On a fresh clone of the repo, I was unable to run pester tests until I manually extracted them the scripts under test. There is some code in Invoke-PesterTests.ps1 that is supposed to unpack any scripts under test, but it does not seem to work properly.

With this change, the code has been reorganized into 3 functions, Import-Pester, Run-Tests and Unpack-Scripts-Under-Test, with the code for Unpack-Scripts-Under-Test reworked the most to make it reliable.

Pre-requisites
[NA] Id should be a GUID that is not 00000000-0000-0000-0000-000000000000
NOTE If you are modifying an existing step template, please make sure that you do not modify the Id property (updating the Id will break the Library sync functionality in Octopus).
[NA] Version should be incremented, otherwise the integration with Octopus won't update the step template correctly
[NA] Parameter names should not start with $
[NA] Step template parameter names (the ones declared in the JSON, not the script body) should be prefixed with a namespace so that they are less likely to clash with other user-defined variables in Octopus (see OctopusDeploy/Issues#2126). For example, use an abbreviated name of the step template or the category of the step template).
[NA] LastModifiedBy field must be present, and (optionally) updated with the correct author
[✔] The best practices documented here have been applied
[NA] If a new Category has been created:
[NA] An image with the name {categoryname}.png must be present under the step-templates/logos folder
[NA] The switch in the humanize function in gulpfile.babel.js must have a case statement corresponding to it

[CLAassistant]

All committers have signed the CLA.

[IsaacCalligeros95]

LGTM, thanks for the contribution!