parted: improve ptest

parted: improve ptest

Add a RRECOMMENDS on kernel-module-scsi-debug as this module is needed
for many of the tests.

Create the udev mount blacklist directory before writing into it, as it
doesn't always exist.

Delete any existing log files before calling make, as otherwise the
tests cannot be repeated.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>