Merge pull request #34407 from dnephin/add-TESTING-doc

[Proposal] Document testing guidelines

CONTRIBUTING.md

@@ -155,10 +155,7 @@ Fork the repository and make changes on your fork in a feature branch:
 	your intentions, and name it XXXX-something where XXXX is the number of the
 	issue.
 
-Submit unit tests for your changes. Go has a great test framework built in; use
-it! Take a look at existing tests for inspiration. [Run the full test
-suite](https://docs.docker.com/opensource/project/test-and-docs/) on your branch before
-submitting a pull request.
+Submit tests for your changes. See [TESTING.md](./TESTING.md) for details.
 
 If your changes need integration tests, write them against the API. The `cli`
 integration tests are slowly either migrated to API tests or moved away as unit
@@ -255,10 +252,9 @@ calling it in another file constitute a single logical unit of work. The very
 high majority of submissions should have a single commit, so if in doubt: squash
 down to one.
 
-After every commit, [make sure the test suite passes]
-(https://docs.docker.com/opensource/project/test-and-docs/). Include documentation
-changes in the same pull request so that a revert would remove all traces of
-the feature or fix.
+After every commit, [make sure the test suite passes](./TESTING.md). Include
+documentation changes in the same pull request so that a revert would remove
+all traces of the feature or fix.
 
 Include an issue reference like `Closes #XXXX` or `Fixes #XXXX` in commits that
 close an issue. Including references automatically closes the issue on a merge.