Move shared test helpers into helper files

Move shared test helpers into helper files

Each helper that more than one test file uses moves out of the test file
that holds it into a file without tests, named for what it holds: the
fake Responses endpoint, the fake ChatGPT login service, the fake Bot
API, the terminal test client, the daemon start with a bot, and the tree
node helpers. A helper that only one test file uses stays in that file.
A reader then finds a shared helper from the file name, and a test file
holds only the tests and the helpers of its own feature.

The code of the helpers does not change.

Issue: 947bddb