Update largefile.test to use default input/output buffer sizes.
Update largefile.test to use default input/output buffer sizes.

The defaults are now sane, and we should test with them.

dbaarda/librsync,sourcefrog/librsync,dbaarda/librsync,sourcefrog/librsync,librsync/librsync,dbaarda/librsync,dbaarda/librsync,sourcefrog/librsync,sourcefrog/librsync,librsync/librsync,librsync/librsync,librsync/librsync