tests: Enable G_TEST_OPTIONS_ISOLATE_DIRS for bookmarkfile test

This ensures that when running many instances of the test in parallel,
they don’t collide in the same current directory, and hence spuriously
fail. This can happen when writing `out.xbel`, for example.

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

Fixes: #1930

glib/tests/bookmarkfile.c

@@ -551,7 +551,7 @@ main (int argc, char *argv[])
   const gchar *name;
   gchar *path;
 
-  g_test_init (&argc, &argv, NULL);
+  g_test_init (&argc, &argv, G_TEST_OPTION_ISOLATE_DIRS, NULL);
 
   if (argc > 1)
     {