docs: readme explains how to run tests (resolves #3639)

[olets]

Description

This PR Adds documentation for how to run the test suite to a new "testing" section in the README.

Motivation and Context

• fix: document a low-friction way to run tests to satisfy PR template checklist item #3639

The pull request template asks contributors to confirm that all tests pass. I could not find documentation for how to run the test suite. Turns out the instructions are pretty clearly implied in the README's "Publishing a release" section. I saw that section's heading and, because I'm not trying to publish, skipped right over it. Can imagine others might too.

Usage examples

During development, relevant for contributors making changes which could affect tests and contributors writing tests.

During proposal: all contributors who open pull requests.

How Has This Been Tested?

I ran the commands listed and confirmed that they ran the test suite.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[escapedcat]

Thanks!