Added basic create and edit post tests.
Added header tests to create and new posts.
Added test for returning a single post.
Added response test for edit context of edit_post endpoint.
Added format, password, date and modified date tests to check_get_post_response.
Create abstract base test class
Check correct date for creation
Check other post types
Add more post creation tests
Use (int) not absint for author sanitization
Check _raw fields with edit context
Add extra braces

This helps coverage understand the code better, and fits with coding
standards.
Create post as test user
Test more post editing
Test post creation permissions
Return 403 error if user can't create posts
Check with invalid post author object
Test creating draft and private posts
Give `null` as date for draft posts
Test password-protected posts