Add runnabletest.AssertCancellation

[pior]

New package runnabletest with AssertCancellation(t, r, wait). Users can check their own runnables against the cancellation contract.

• Two subtests: cancelled while running (after 10ms), and an already-cancelled context.
• Passes on nil or context.Canceled (matched with errors.Is, like the Manager). Fails on any other error, including context.DeadlineExceeded, and when the runnable doesn't return within wait.
• Uses only the standard library. The failure cases are tested by running them in a child test process.
• The internal AssertRunnableRespect* helpers stay, because package runnable tests can't import runnabletest (import cycle).