Added test for repository-set - available repository

 Added test for repository-set - available repository

please review it
thanks
Via IRC:

    <tkolhar> ichimonji10, yes actually some more code can be added . but manually it failing # hammer repository-set available-repositories --name orac_repo --organization Default_Organization --product prod
    <tkolhar> [Foreman] Username: ***
    <tkolhar> [Foreman] Password for ***:
    <tkolhar> undefined method `size' for nil:NilClass

In this case, maybe something like `self.fail('This stubbed test should be fleshed out')` should be used as the test body instead of `pass`? That way, as soon as the BZ bug is fixed, our attention will immediately be drawn back to that code b/c it's failing. What do others think?