Pacify recent warnings in ansi-color-tests.el

Pacify recent warnings in ansi-color-tests.el

* test/lisp/ansi-color-tests.el: Remove invalid package keyword
'ansi', and unused dependency on cl-lib.
(yellow, bright-yellow): Replace prefix-less dynvars with
corresponding local lexvars.
(test-strings): Rename prefix-less dynvar...
(ansi-color-tests--strings): ...to this.  All users updated.
(ansi-color-apply-on-region-test)
(ansi-color-apply-on-region-bold-is-bright-test): Simplify.
(ansi-color-apply-on-region-preserving-test): Reindent.