DebugByDefault changes the default of the `-fuse.debug` flag to true.
This package registers a command line flag `-fuse.debug` and when run with that flag (and activated inside the tests), logs FUSE debug messages.
This is disabled by default, as most callers probably won't care about FUSE details. Use DebugByDefault for tests where you'd normally be passing `-fuse.debug` all the time anyway.
Call from an init function.