doc: update definition of tests #1732

Background

We agreed in August, to use the following terminology when referring to tests:

• Unit test -> rust tests in /src folder [0]
• Integration test -> rust tests in /tests folder per crate [1]
• System test -> pytest

c.f. https://nearone.slack.com/archives/C07UW93JVQ8/p1756219425948399

Unfortunately, our docs don't reflect that change, which recently lead to some confusion related to an issue:
#1725 (comment)

User Story

As a dev, I might not have context from different sources and would like the repo to be self-explanatory.

Acceptance Criteria

Update the readme such that it reflects the agreed-upon defintion.

Resources & Additional Notes

No response