Create testing documentation

Some untold processes for testing like:
1. running tsc after changing the runners
2. Running `npm test `
3. Document the way the custom transformers and runners work (big files with no tests?)
etc.

Use the listed testing entry points first: run tsc after runner changes and try npm test to confirm the workflows that need documenting. Then inspect the custom transformers and runners, including the large files noted as lacking tests; done means the testing processes and their operation are documented.

Written by the indexing model from the issue text.