tests: remove use of unprotected 'set'

tests: remove use of unprotected 'set'

* cfg.mk (sc_prohibit_bare_set): A new syntax check to
ensure we protect use of set with '--', so that args
beginning with '-' are not interpreted as options,
and if no args are present, all existing args are cleared.
* tests/cp/symlink-slash.sh: Add -- to unprotected use of set.
* tests/ls/ls-time.sh: Likewise.
* tests/ls/symlink-slash.sh: Likewise.
* tests/mkdir/perm.sh: Likewise.
* tests/mkdir/selinux.sh: Likewise.
* tests/mkdir/smack-no-root.sh: Likewise.
* tests/mkdir/smack-root.sh: Likewise.
* tests/mv/part-hardlink.sh: Likewise.
* tests/nice/nice.sh: Likewise.
* tests/stty/stty-row-col.sh: Likewise.