Test policy documentation is missing

[tuminoid]

Requirement:

The project MUST have evidence that the test_policy for adding tests has been adhered to in the most recent major changes to the software produced by the project. [tests_are_added]

and

It is SUGGESTED that this policy on adding tests (see test_policy) be documented in the instructions for change proposals. [tests_documented_added]

In other repos, we have docs/testing.md.

[Rozzii]

/triage accepted
/kind documentation