Rename incorrect `post_excerpt` argument in User fixture

[over-engineer]

Description

Closes #99 - renames the post_excerpt user data argument to description in the User fixture.

Change Log

• Rename post_excerpt to description in the User fixture.

Types of changes (if applicable):

• [ ] Bug fix (non-breaking change which fixes an issue).
• [ ] New feature (non-breaking change which adds functionality).
• [x] Breaking change (fix or feature that would cause existing functionality to not work as expected).

Checklist (if applicable):

• [x] Meets provided linting standards.

[over-engineer]

Hey! 👋

This PR changes a single word – post_excerpt to description. With that being said, this could potentially be a breaking change because all users will be generated with a (biographical) description. That might break things for a test that depends on a user not having a description (i.e. the default behavior up until now).

If you believe this change might be risky, I'd suggest removing the post_excerpt from the user's default values altogether since it's not applicable to a User fixture (see wp_insert_user()).