Package centraltest gives a test a muster in front of a beekeeper serve:
its call_tool forwards x_<server>_<tool> to the backend with the caller's
bearer and wraps the result as muster does, and a muster binary that
names its endpoint and prints a token.
Binary writes a muster binary for one person: `context show` names
endpoint, `auth token` prints the token file's content. It returns the
binary and the token file, which the test may rewrite.