Add test "t.cp-a".

I've just had to debug a difficult problem in genimage tests (See https://github.com/pengutronix/genimage/issues/37 ) and I would like to add a unit test to fakeroot that catches this problem when it arises.

It arises with coreutils 8.30's "cp -a". If using fakeroot, it will not copy the UNIX mode to the UNIX mode but rather create a new ACL with it in the target. Otherwise, it works just fine.