I've opened issue 12652 to track this task.
1. First, the devguide incorporates the documentation
2. Then, I'll clean up the official docs and add pointers to the devguide
The transition of the documentation into the devguide also has a few steps:
1. Move the reference of the utilities test.support provides. This reference
documentation isn't complete (not all functions are documented), but can be
completed later in the devguide.
2. (Optionally) incorporate the test-writing advice given in 25.5.1 into the
devguide (which currently has only a short couple of paragraphs
3. Move any other information from the official docs (chapter 25) into the
devguide, leaving only a section on running the tests (which should be
identical to the relevant section in the devguide) and a pointer to the
Issue #12652 now has a patch for step (1) above.