Allow event-loop signal tests to pass on FreeBSD

On Linux the signal will be immediately visible in the epoll_wait() call.
However, on FreeBSD we may need a small delay between kill() call and the
signal being visible to the kevent() call. This sometimes happens when the
signal processing and kevent processing runs on different CPUs in the
kernel, so becomes more likely when the system is under load (e.g. running
all tests in parallel).

See https://github.com/jiixyj/epoll-shim/pull/32



Signed-off-by: Alex Richardson <Alexander.Richardson@cl.cam.ac.uk>