; completion-preview-tests.el: Deduplicate some test code

; completion-preview-tests.el: Deduplicate some test code

* test/lisp/completion-preview-tests.el
(completion-preview-tests--insert-and-preview): New function.
(completion-preview)
(completion-preview-multiple-matches)
(completion-preview-exact-match-only)
(completion-preview-function-capfs)
(completion-preview-non-exclusive-capfs)
(completion-preview-face-updates)
(completion-preview-capf-errors)
(completion-preview-complete)
(completion-preview-insert-calls-exit-function): Use it.