Enhance UT's for GitHubRepositories.ps1

[HowardWolosky]

We use a number of the functions within this file throughout the other UT's, but not all of them. There is a lack of comprehensive testing available for all the methods within this file as was recently demonstrated with #134 and #136 (around Update-GitHubRepository).

We should close the gaps on the testing of the functions within this file to increase code coverage and reduce regression possibilities.

[giuseppecampanelli]

I'll have a go at this