tests: Use pgrep instead of ps

tests: Use pgrep instead of ps

The ps command as shipped by busybox does not support the "command"
column. Change the code to use pgrep instead which will work fine in all
environments.

In addition, raise an exception if PID was not found since the test
would just hang with pid = 0.

Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>