feat: update testing db instructions lesson

[01zulfi]

Because

#27928

This PR

replaces MongoDB instructions with how to setup test postgres db. Touch a bit on unit & integration test

admittedly the lesson is a bit undercooked (the whole testing section needs a overhaul). this PR just gets enough stuff done to push node revamp milestone 1 out the door

Issue

Closes #27928

Additional Information

Pull Request Requirements

• I have thoroughly read and understand The Odin Project curriculum contributing guide
• The title of this PR follows the location of change: brief description of change format, e.g. Intro to HTML and CSS lesson: Fix link text
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• If this PR addresses an open issue, it is linked in the Issue section
• If any lesson files are included in this PR, they have been previewed with the Markdown preview tool to ensure it is formatted correctly
• If any lesson files are included in this PR, they follow the Layout Style Guide

[MaoShizhong]

🔥

[MaoShizhong]

@01zulfi What do you think about assignments? Is there anything e.g. documentation that you think may be worth mentioning in a short assignment section?

[01zulfi]

@MaoShizhong I have nothing in mind right now. We'll not be merging this PR until we ship. So, I might add an assignment item or two after working through the Prisma lessons and revising all of the other lessons.

[MaoShizhong]

@MaoShizhong I have nothing in mind right now. We'll not be merging this PR until we ship. So, I might add an assignment item or two after working through the Prisma lessons and revising all of the other lessons.

That's fair. I think having a brief assignment section would benefit, on top of following the style guide of course. Not 100% sure what since I'm less familiar with the new content specifically, but I'll drop any suggestions if I come across them.