quilt: run tests as ptest user, and let that user write into the tests

The tests need to run as a non-root user, but that user also needs to be
able to write into the tests directory to create the work directories.

Use the new ptest user that ptest-runner creates to avoid having to add
a new user, and ensure that the test/ directory is owned by the ptest
group and that group has write access.

This solves the ptest regression where since oe-core 9a287b45 no quilt
tests were actually running.

A better solution would be to allow the tests to create a work directory
in another directory, but that would need work upstream which can happen
later.

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