tests: remove flakiness in test-nointerrupt.t
The problem was that the reaction to the signal was racing against
the completion of the command.
Since reaction to the signal is to print a line of warning,
we can fix this by waiting for that warning to appear before
allowing the command to complete.
from typing import Optional
version: int
def b85encode(text: bytes, pad: Optional[int]) -> bytes: ...
def b85decode(text: bytes) -> bytes: ...