lib/logger: Clean up LOGGER_DISCARD handing to unbreak tests
lib/logger: Clean up LOGGER_DISCARD handing to unbreak tests

When using newLogger() with an io.Writer, that writer should in fact be
used...

syncthing/syncthing,wweich/syncthing,xduugu/syncthing,imsodin/syncthing,syncthing/syncthing,imsodin/syncthing,xduugu/syncthing,wweich/syncthing,imsodin/syncthing,imsodin/syncthing,syncthing/syncthing,xduugu/syncthing,wweich/syncthing,AudriusButkevicius/syncthing,syncthing/syncthing,imsodin/syncthing,syncthing/syncthing,wweich/syncthing,AudriusButkevicius/syncthing,AudriusButkevicius/syncthing,AudriusButkevicius/syncthing,syncthing/syncthing,xduugu/syncthing