* test/automated/cl-lib-tests.el: Add tests for plusp, second, ...

* test/automated/cl-lib-tests.el: Add tests for plusp, second, ...

(cl-lib-test-plusp, cl-lib-test-minusp)
(cl-lib-test-oddp, cl-lib-test-evenp, cl-lib-test-first)
(cl-lib-test-second, cl-lib-test-third, cl-lib-test-fourth)
(cl-lib-test-fifth, cl-lib-test-sixth, cl-lib-test-seventh)
(cl-lib-test-eighth, cl-lib-test-ninth, cl-lib-test-tenth)
(cl-lib-test-endp, cl-lib-test-nth-value)
(cl-lib-nth-value-test-multiple-values, cl-test-caaar, cl-test-caadr)
(cl-test-ldiff): New tests.
(cl-digit-char-p): Tighten the test.