Fixes #6620: Allow plugins to define a file to be included in test_helper.


For some plugins additions to the test_helper are needed to allow
the test suite to pass. This checks if a file test/support/foreman_support
exists and requires it after all the Foreman test setup is done.
I am OK with that name change. I could even make it more explicit with something like `foreman_test_helper_additions.rb`.