tests: Allow FLATPAK_BINARY=flatpak for exports

https://github.com/flatpak/flatpak/commit/a99b748931e6444dadb0d0c8c1565da23f755800 introduced an environment variable for changing the flatpak path for exports.
On NixOS, we do not have a stable path we could use so we used to patch Flatpak to use `flatpak` program from `PATH`.
With the recent change, we drop our downstream patch in favour of setting the environment variable to `flatpak` but the tests do not expect that.

This is the test part of our former downstream patch so that tests can pass with exports using `flatpak` instead of an absolute path.
It still expects the binary path to end with `flatpak` so arbitrary `$FLATPAK_BINARY` will not work but we do not pass the environment variable to tests so we cannot do much better.

(cherry picked from commit 96dbe28cfa96e80b23fa1d8072eb36edad41279c)