docs: enhance the test reporter's README

[galargh]

• Because this PR includes a bug fix, relevant tests have been included.
• Because this PR includes a new feature, the change was previously discussed on an Issue or with someone from the team.
• I didn't do anything of this.

---

This PR enhances the README of the node test reporter. It is related to #5437.

My aim in this version was to:

• understand the current implementation of the package
• describe the current state of the package in the README (highlighting sections relevant to potential users)

As further enhancements to the reporter are introduced, the README should be updated accordingly.

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
hardhat	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Aug 23, 2024 8:49pm

[changeset-bot]

⚠️ No Changeset found

Latest commit: 0c1d97d

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR