docs(NODE-3753): Update testing readme file

[ljhaywar]

Description

NODE 3753

What is changing?

Updating the Test readme to have more detailed information about the tests

Is there new documentation needed for these changes?

No

What is the motivation for this change?

The readme was lacking details that newbies need to understand the test infrastructure

Double check the following

• Ran npm run check:lint script
• Self-review completed using the steps outlined here
• PR title follows the correct format: <type>(NODE-xxxx)<!>: <description>
• Changes are covered by tests
• New TODOs have a related JIRA ticket

[ljhaywar]

Some notes...

Benchmark tests explanation - this was very much an educated guess. Is there any more info on those that we can point people to?

Using a Pre-Release Version of a Dependent Library. I based the instructions on Daria’s comments in the Jira ticket. I was able to follow the steps, and the tests didn’t blow up, but I wasn’t sure how to verify I was actually using the pre-release version.

I left TODOs for explaining what the environment variables represent and how to get their values. If someone wants to take a crack at explaining that, I’m happy to clean up the wording.

I left “TODO Special Env Sections” alone. If someone wants to take a crack at those, I’m happy to clean up the wording.

[ljhaywar]

@durran Thanks for the feedback! I've made updates. Can you take another look at your convenience?

[nbbeeken]

Thanks so much! this is a great write up 😃

[dariakp]

This is great, thank you so much for the work you did here!

[baileympearson]

The changes look good to me (given my limited experience) and they've already proved helpful. Thanks for taking this on! 😃