A race may occur between the router process restart and the main process
sending a notification to the running controller. For example, a test script
detects the new process and starts performing a smoke test, but the controller
has not yet received the 'remove PID' notification, so the connection to the
router is broken and any attempt to update the configuration will cause an
error.
The solution is to perform several attempts to reconfigure Unit with a short
delay between failures.
Running `test_respawn_` test cases on a machine with Unit
daemon in background would fail tests because `ps ax` was
used without filtering out other unit instances.
This patch also prevents from tests killing other Unit
processes not related to tests.
