tests: Fix test-summaries.sh when running tests as root
tests: Fix test-summaries.sh when running tests as root

If the tests are run as root, the system helper is not used even when
commands are run with `--system`. This means a different cache path is
used (see `_flatpak_dir_ensure_repo()`). The tests did not accommodate
for that.

Signed-off-by: Philip Withnall <12d956b70be7f0d60e54e48e97a01dac38dce989@endlessos.org>

flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak