Update CommentTestBase::setCommentPreview to use CommentPreview enum

Problem/Motivation

Follow-up to 📌 Each class using DRUPAL_* constants from system.module should define its own constants Needs work

We will have an enum for CommentPreview states which can be used in CommentTestBase::setCommentPreview

Proposed resolution

Not sure if we need to maintain BC on core test bases? Ideally not.

- Update $mode param to take CommentPreview
- Update switch statement to a match
- Pass $mode->value into setCommentSettings

Remaining tasks

Do it

API changes

CommentTestBase::setCommentPreview now takes a CommentPreview object instead of int for $mode

Release notes snippet