Backport to v2019 the disabling of gpg tests

This is a backport of the fix we already added to v2021dev2 version of flatpak.

To quote the reason: Disable failing tests during build because the version of Apertis in OSTree does not include GPG support because of GPLv3. Thus, when the flatpak tests are run, libostree returns with "no gpg support" without setting the GError