Meson: add tests to validate appdata/desktop file
This MR fixes my latest broken one.
It also avoid this kind of issues by using a project_id var.
bilelmoussaoui@localhost ~/gnome-tweaks> ninja -C _build test ninja: Entering directory `_build' [2/3] Running all tests. 1/2 Validate appdata file OK 0.02 s 2/2 Validate desktop file OK 0.01 s Ok: 2 Expected Fail: 0 Fail: 0 Unexpected Pass: 0 Skipped: 0 Timeout: 0