tests: Isolate directories in appinfo test
tests: Isolate directories in appinfo test

This is essentially a reversion of commit
a39b847ddf3fe63fd5f7e1134b46fc888c10df5e, plus the addition of the new
G_TEST_OPTIONS_ISOLATE_XDG_DIRS option.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://gitlab.gnome.org/GNOME/glib/issues/1601

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib