bluez/ubt: Fix test_dun_pan

The result of an urlrequest.read is a binary stream of data.
The input for file.write() is a string.
Make sure the write call is fed with the proper type of data.

Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>
Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>
Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>
Signed-off-by: Frédéric Dalleau <frederic.dalleau@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D4446