Add more completion tests

Add more completion tests

* test/lisp/minibuffer-tests.el (completion--pcm-score)
(completion--pcm-first-difference-pos): New helpers.
(completion-pcm-test-1, completion-pcm-test-2)
(completion-pcm-test-3, completion-pcm-test-4)
(completion-pcm-test-5, completion-pcm-test-6)
(completion-substring-test-1, completion-substring-test-2)
(completion-substring-test-3, completion-substring-test-4)
(completion-flex-test-1, completion-flex-test-2)
(completion-flex-test-3): New tests (bug#42149)