Fix DisconnectTest for devices with large socket buffers

Similar to commit e3bfefb1
but for DisconnectTest. The buffer sizes are large on some
devices so the expectation that they take a lot of time to
stream data is invalid because they just write into the
buffers.

Bug: 18703696
Change-Id: I6127b044bf7fefc192f577d0b035deacbc54dd5f