Fix test failures in test/lisp/minibuffer-tests.el

Fix test failures in test/lisp/minibuffer-tests.el

bug#48841, bug#47711

In some instances the test code needed to be updated to make different
assumptions about implementation details.

In others, like the ones about the completions-first-difference face,
minor parts of the actual user-visible behaviour were broken.

* test/lisp/minibuffer-tests.el (completion-test1): Robustify test.
(completion--pcm-score): Don't assume completion-score is stored
in string as a property.

* lisp/minibuffer.el (completion--hilit-from-re): Take new parameter.
(completion-pcm--hilit-commonality): Use it.