tests: cp/perm (usually not run) was failing on systems with SELinux

tests: cp/perm (usually not run) was failing on systems with SELinux

* tests/cp/perm: Use stat to get the permission string, not ls.
This test was run only when RUN_VERY_EXPENSIVE_TESTS=yes was set
in the environment.  It would fail on SELinux-enable systems
because ls-generated permission strings would not match, e.g.,
"test _-rw-r--r--+ = _-rw-r--r--" would fail.