Fix up the appinfo test

One testcase was launching appinfo-test from a GAppInfo that
does not have a filename. In this case, the G_LAUNCHED_DESKTOP_FILE
envvar is not exported. Make appinfo-test deal with that, without
spewing warnings.

https://bugzilla.gnome.org/show_bug.cgi?id=711178
Fix up appinfo test

Make it possible to skip the terminal-launching test simply
by setting DISPLAY= . Previously, you had to unset DISPLAY,
which is a little more cumbersome.

https://bugzilla.gnome.org/show_bug.cgi?id=711178