Stop any spawned ProcessMonitor at test cleanup

Stop any spawned ProcessMonitor at test cleanup

Base test class adds a cleanup fixture to stop any
spawned process monitor via unit or functional tests,
which otherwise would keep running after the tests already
finished, and execution functions go unpatched.

Without this patch unit tests will randomly fail
when we enable process monitor by default at change:
I85fe31bee30714148168a293eea29fa0a37f9701

Co-Authored-By: Maru Newby <marun@redhat.com>
Change-Id: Ide799a52391b14ff921de25788e8b0f0463fb8f8