ptest-runner: add non-root ptest user for tests to run as

Some tests really don't want to be run as root, either because they
expect to get "permission denied" errors occasionally, or have explicit
checks.

Instead of numerous recipes re-inventing user creation in run-ptest,
create a user in the ptest-runner recipe that they can all use.

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