testtreelistmodel: Don't use non-standard function

as per efd3758f6a strcasecmp() is not a C
standard thing (not that we bothered including any header for it anyway)
and so this test failed to build on Windows with Microsoft Visual C.

tests/testtreelistmodel.c

@@ -131,7 +131,7 @@ compare_files (gconstpointer first,
   first_path = g_file_get_path (first_file);
   second_path = g_file_get_path (second_file);
 
-  result = strcasecmp (first_path, second_path);
+  result = g_ascii_strcasecmp (first_path, second_path);
 
   g_free (first_path);
   g_free (second_path);