Codebase Unit Tests

## Description
Currently, the codebase does not have many unit tests. We want to add unit test coverage using `pytest` for at least every `extensions`.

**Extensions**:
- [ ] Bookmark Cog
- [ ] Code Generator Cog
- [ ] Color Cog
- [ ] Extension Cog
- [ ] Fun Cog
- [ ] Grace Cog
- [ ] Language Cog
- [ ] Mermaid Cog
- [ ] Moderation Cog
- [ ] Pun Cog
- [ ] Reddit Cog
- [x] Reminder Cog
- [ ] Thank Cog
- [x] Threads Cog
- [x] Time Cog
- [ ] Translator Cog
- [ ] Weather Cog
- [ ] Welcome Cog
- [ ] Wikipedia Cog

> Anyone feel free to take on a cog listed above. Make sure to mention it here or on Discord to avoid multiple people working on the same test. I will update the list once the unit tests are merged.

Thank you to anyone contributing to this update.