Fix slug behaviour

[cojennin]

Another take on fixing slug issues in #523

Added a bunch more tests, both around wp_insert_post and the Rest API to give a bit more confidence to this PR.

To Test

The following should all be captured in the automated tests, but for manually testing:

Do this for posts and pages

In Classic Editor & In Block Editor

1. Create a post, save it as a custom status
2. Update the post title
3. Confirm the post slug has been updated

In Classic Editor & In Block Editor

1. Create a post, save it as a custom status
2. Set the post slug and save the post
3. Update the post title and save the post
4. Confirm the slug has not changed

In Classic Editor & In Block Editor

1. Create a page, save it as a custom status
2. Update the page title
3. Confirm the page slug has been updated

*Repeat this but make the page a child page

In Classic Editor & In Block Editor

1. Create a page, save it as a custom status
2. Set the page slug and save the post
3. Update the page title and save the post
4. Confirm the slug has not changed

*Repeat this but make the page a child page

[cojennin]

This PR is already being used by other folks, so if no one has anything they want to flag I'll merge as is