docs: adds instructions on how to run unit and e2e tests to the README

[krpeacock]

Description

As requested in https://github.com/dfinity/agent-js/pull/877, this PR adds improved documentation on how to run tests in the agent-js repo during local development

Checklist:

• [x] My changes follow the guidelines in CONTRIBUTING.md.
• [x] The title of this PR complies with Conventional Commits.
• [x] I have edited the CHANGELOG accordingly.
• [x] I have made corresponding changes to the documentation.

[github-actions[bot]]

size-limit report 📦

Path	Size
@dfinity/agent	85.62 KB (0%)
@dfinity/candid	13.58 KB (0%)
@dfinity/principal	4.97 KB (0%)
@dfinity/auth-client	60.71 KB (0%)
@dfinity/assets	80.24 KB (0%)
@dfinity/identity	57.92 KB (0%)
@dfinity/identity-secp256k1	265.65 KB (0%)