Test the retry helper

retry_call(func, attempts, sleep) is shared by every service, so its contract must be pinned by tests: it returns func()'s result; it retries only TimeoutError, calling sleep(2**n) between tries (1, 2, 4 …); it re-raises the last TimeoutError after attempts tries; any other exception is raised immediately without sleeping. Write tests (unittest or test_ functions) that pass for a correct helper and fail for broken ones. Fake func and sleep with small functions or unittest.mock.Mock; nothing should really sleep. Leave the calls to Check.