Need Testing.md

We need a document for developers explaining how to run the testsuite, how and when to update test reference output, how to add new tests or approach testing for new features, etc.

Start by locating the repository’s existing developer documentation and test-suite entry point. The work is complete when a Testing.md document explains how to run tests, when to update reference output, and how to add or approach tests for new features.

Written by the indexing model from the issue text.