Fix the logic for skipping _repo_setup_updates
Just checking for the package update list is wrong in a couple of
ways:
1. It goes wrong for child tests that inherit the main disk from
the parent test, but not the second disk where the actual update
repo lives. They'll have the package list, but not the repo. So
we should test for both the package list and the repo dir.
2. It goes wrong for side tag tests, because we don't write a
package list (or create an update repo) on that path. So on that
path let's check whether we already created the repo definition
we use on that path.
Signed-off-by: Adam Williamson <awilliam@redhat.com>