for gimp-tests.log
On Linux we may not have permission to create a log at our default
location, so we need to check for that.
First we should not init the log when our plug-in is asked for info.
Only create it when we actually run the import or export tests.
Second, try to catch the permission error and write a message either
in GIMP or in the terminal for non-interactive.
97671dbab7