add MacOs pidof compatibility

Summary:
Adds MacOs pidof compatibility for bitcoind detection.

Otherwise, the warning is always shown:
WARNING! There is already a bitcoind process running on this system. Tests may fail unexpectedly due to resource contention!

if pidof: stable 0.1.4 (bottled) from brew is installed.

Depends on D999

Test Plan: ./test/functional/test_runner.py

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D1008