Fix a race in the kqueue-read test. If the writer thread managed to fill

Fix a race in the kqueue-read test. If the writer thread managed to fill
up the pipe buffer, it will hang in write(2) waiting for the pipe to
become writeable again. While doing so, it cannot observe that the
kqueue thread as already finished implying that ctx_thread_alive() would
return false, meaning no further writes are wanted.

Therefore open the pipe in non blocking mode and just ignore EAGAIN
errors while writing.