Tests: Swap usages of pidof for pgrep --full

Some implementations of pidof (such as the one from procps-ng)
seem immune to changing a process' name using prctl() and
overwriting argv[0]. Using preg --full works around this
problem.

In time, we should ensure every deamon publishes a PID file
which can be reliably used by the tests.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>