Stop reimplementing a bunch of cl- functions in ert

Stop reimplementing a bunch of cl- functions in ert

* lisp/emacs-lisp/ert.el: Require cl-lib at runtime too.
(ert--cl-do-remf, ert--remprop, ert--remove-if-not)
(ert--intersection, ert--set-difference, ert--set-difference-eq)
(ert--union, ert--gensym-counter, ert--gensym-counter)
(ert--coerce-to-vector, ert--remove*, ert--string-position)
(ert--mismatch, ert--subseq): Remove reimplementations of cl funcs.
(ert-make-test-unbound, ert--expand-should-1)
(ert--expand-should, ert--should-error-handle-error)
(should-error, ert--explain-equal-rec)
(ert--plist-difference-explanation, ert-select-tests)
(ert--make-stats, ert--remove-from-list, ert--string-first-line):
Use cl-lib functions rather than reimplementations.

* test/automated/ert-tests.el: Require cl-lib at runtime too.
(ert-test-special-operator-p): Use cl-gensym rather than ert-- version.
(ert-test-remprop, ert-test-remove-if-not, ert-test-remove*)
(ert-test-set-functions, ert-test-gensym)
(ert-test-coerce-to-vector, ert-test-string-position)
(ert-test-mismatch): Remove tests.

* test/automated/cl-lib.el: New, split from ert-tests.el.