cpio: improve ptests

cpio: improve ptests

We don't need to run 'make check' across the source tree.

Forcibly regenerate tests/testsuite so it supports the --am-fmt option
that we patch into autoconf.

Add coreutils to ptest RDEPENDS as the symlink-bad-length test needs
base64.

Clean up run-ptest: there's no need to run atconfig manually, and use
--am-fmt instead of manually sedding the output.

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