g_desktop_app_info_load_from_keyfile() refuses to load .desktop files
where the executable doesn't exist. Therefore whether or not the .desktop
file added in commit 148995544 is actually considered during tests depends
on /usr/bin/flatpak being installed. This isn't a safe assumption to make,
so use /bin/sh to test filtering of "prefix" commands.
https://bugzilla.gnome.org/show_bug.cgi?id=795488
Add the basename from the first component of the Exec line to the list of
strings to search for via g_desktop_app_info_search().
We treat Exec as a fairly strong match -- just below the visible name.
Add a testcase to make sure everything is working OK.
https://bugzilla.gnome.org/show_bug.cgi?id=725023
