tests: adjust PATH to generally include /sbin and /usr/sbin

tests: adjust PATH to generally include /sbin and /usr/sbin

Commit 5eeaca94 added /sbin to the PATH for tests using mkfs. For other
tests, e.g. tests/cp-fiemap-perf using filefrag, we need /usr/sbin also.
Add both directories generally for the tests, "since many of us always
augment our PATH with all of the sbin paths all of the time anyway" (Bob
in http://lists.gnu.org/archive/html/coreutils/2011-11/msg00107.html).
The previous commit is hereby obsolete.
* tests/init.cfg (sanitize_path_): Add /sbin and /usr/sbin to PATH
unless already included. Needed for tests using admin tools like mkfs
and filefrag on systems where the user's environment does not have
these directories in the PATH.
* tests/init.cfg (require_mkfs_PATH_): Remove obsolete function.
* tests/cp/cp-a-selinux: Remove require_mkfs_PATH_ call.
* tests/cp/cp-mv-enotsup-xattr: Likewise.
* tests/cp/sparse-fiemap: Likewise.
* tests/mkdir/writable-under-readonly: Likewise.
* tests/rm/read-only: Likewise.