tests: Skip appinfo/associations test if update-*-database not installed

If `update-desktop-database` or `update-mime-database` are not
installed, there are a few `GAppInfo` methods for modifying file
associations which will print a `g_warning()` when called. Something
like:
```
GLib-GIO-FATAL-WARNING: Failed to execute child process ‘update-desktop-database’ (No such file or directory)
```

(Example: https://gitlab.gnome.org/zamaudio/glib/-/jobs/3190053)

This will cause the appinfo/associations test to fail, as warnings are
fatal.

If that’s going to happen, skip the test. We can’t hard-depend on these
tools as they are external to GLib and only needed for a few operations.
Instead we have a soft runtime dependency on them; that should be
reflected in the tests.

Signed-off-by: Philip Withnall <philip@tecnocode.co.uk>

Helps: #3148

gio/tests/meson.build

@@ -330,8 +330,6 @@ if host_machine.system() != 'windows'
       'appinfo' : {
         'install' : false,
         'extra_programs' : ['appinfo-test'],
-        # FIXME: https://gitlab.gnome.org/GNOME/glib/-/issues/3148
-        'can_fail' : host_system == 'gnu',
       },
       'desktop-app-info' : {
         'install' : false,