flatdeb-demo: add remote using .flatpakrepo file
Our Flatpak repo now implements ED25519 signatures and provides a
.flatpakrepo file for making it easier to configure the remote,
including signature verification. Switch this test case to using this
method.
Other Flatpak-related test cases use the flathub repo and therefore
should be left untouched for now.
Edited by Andre Moreira Magalhaes