test: use pgrep for better compatibility

pidof is not available on BSD system, while pgrep is present on BSD, Linux and macOS

test: retry when write to a socket fails on macOS

If the socket is tearing down macOS will return EPROTOTYPE instead of EPIPE.
Because python doesn't handle this internally we have to do a workaround and retry the request.
See https://bugs.python.org/issue33450

This is a backport of core#19368

test/functional/test_runner.py

Run it with and without bitcoind already running to check that it still detects it.