cpio: Run ptests under ptest user

Running these tests under root user on musl works ok but it emits
additional diagnostic

cpio: dir: Cannot change ownership to uid 0, gid 0: Not supported

Ideally its better to run this test suite under non-root user which
is often how these are run. Moreover, tests work on musl as well.

(From OE-Core rev: 22d19266269c28f596f031fcba8f2831c6465880)

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>