[test][bcp] Do not use qsize as it is not implemented on macOS

Store the returned value and check that we have what we
wanted.

https://docs.python.org/3/library/multiprocessing.html?highlight=qsize
2 jobs for zmq_refactoring in 17 minutes and 37 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Build
passed #157767
docker-build
build_linux_36

00:17:37

65.0%
passed #157768
macosx
build_macosx_36

00:15:06

65.0%