tests: adjust PATH to include /sbin for mkfs-using tests

tests: adjust PATH to include /sbin for mkfs-using tests

* tests/init.cfg (require_mkfs_PATH_): New function to test whether mkfs
is in PATH, otherwise adding /sbin to PATH.  Needed for distributions
(OpenSuSE, Solaris) in which sudo does not include /sbin in PATH.
* tests/cp/cp-a-selinux: Use require_mkfs_PATH_.
* tests/cp/cp-mv-enotsup-xattr: Likewise.
* tests/cp/sparse-fiemap: Likewise.
* tests/mkdir/writable-under-readonly: Likewise.
* tests/rm/read-only: Likewise.