Use the new `locate_test_data()` helper to get access to test-data.
Guard the test with `have_test_data()` to skip it in case test-data
access is not available.
This adds two more os-release tests. One contains an empty os-release
file. We verify it is correctly parsed and ends up with the default
value.
The second one is the official Arch Linux os-release file. We verify
that we correctly end up with the rolling-release name.