gio: test that launch_uris() exports files with the document portal when launching a flatpak

While working on !1067 (closed), there was a request to add some tests. There were no tests for the existing Flatpak support code, so I started by writing a test for that case.

As the gdocumentportal code stores a reference to the GDBusConnection in a global, I've adjusted the tear down of the test to just call session_bus_stop, since the reference count on the connection will never drop to zero.