tests: Fix desktop-app-info test

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

gio/tests/desktop-files/usr/applications/org.gnome.clocks.desktop

@@ -342,7 +342,10 @@ Keywords[zh_HK]=time;timer;alarm;world clock;stopwatch;time zone;時間;計時
 Keywords[zh_TW]=time;timer;alarm;world clock;stopwatch;time zone;時間;倒數;計時器;鬧鐘;鬧鈴;世界時鐘;碼表;時區;
 Keywords[ug]=time;timer;alarm;world clock;stopwatch;time zone;ۋاقىت;ئۆلچىگۈچ;قوڭغۇراق;دۇنيا سائىتى;ۋاقىت رايونى;
 Keywords=time;timer;alarm;world clock;stopwatch;time zone;
-Exec=/usr/bin/flatpak run --branch=stable --arch=x86_64 --command=gnome-clocks org.gnome.clocks
+# GDesktopAppInfo requires that the command exists, so use /bin/sh
+# rather than /usr/bin/flatpak, since that's guaranteed to exist on
+# all test systems (in particular CI machines).
+Exec=/bin/sh run --branch=stable --arch=x86_64 --command=gnome-clocks org.gnome.clocks
 Icon=org.gnome.clocks
 Terminal=false
 Type=Application