There seems to be no reason to do so, and since the `appinfo` test was
ported to use `G_TEST_OPTION_ISOLATE_DIRS`, it has been causing
coredumps to accumulate. `gnome-terminal` was chosen as the terminal,
but it couldn’t find its GSettings schemas due to all the XDG
environment variables being cleared to `/dev/null` by
`G_TEST_OPTION_ISOLATE_DIRS`.

In order to keep using `gnome-terminal` as a subprocess in the tests,
we’d need to explicitly set up its environment so it can load the right
GSettings schemas. That’s a lot of work for not much gain.

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

Helps: #436