test: ☑️ refactor post model tests

[rodneylab]

Description

Refactor post model tests into test app to help write cleaner tests going
forward.
Include database pool generator in TestApp.

Type of change

• Test

How Has This Been Tested?

• cargo test run with all tests passing

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules