bluez/ubt: Use temporary filename in dun and pan tests

In dun and pan tests, a file is retrieve from http to verify connection,
it is saved at the location defined in variable path. But path do not
contain a file path. Instead it's a dbus object path in /net/connman
namespace. As such, it is not suitable for using as a filename and any
tentative to open this file will end with an "File Not Found" error.

Instead, create a temporary filename that will be automatically deleted.

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/D4447