tests: start the PermissionStore before the xdg-desktop-portal
tests: start the PermissionStore before the xdg-desktop-portal

This fixes a race condition: previously, the portal would implicitly
start the permission store via StartServiceByName. By the time we
started the permission store manually with --replace, the bus name was
already owned by that implicitly started one.

We check for a name owner after starting the permission store but not
whether that name owner is our manually started one. So we'd
receive the ok for the original owner, complete global_setup() and start
the tests. global_setup() also initializes the proxy variable
"permission_store" which all tests rely on - but that points
to implicitly started one.

Eventually, the manually started permission store replaces the
implicitly started one on the bus and our proxy variable no longer
points to the PermissionStore and tests fail.

flatpak/xdg-desktop-portal,flatpak/xdg-desktop-portal,flatpak/xdg-desktop-portal,flatpak/xdg-desktop-portal