Fix race in TestDaemonRestartWithVolumesRefs

Sometimes rm begins before process death, but Kill called already after
it, so we get error - no such process.

Signed-off-by: Alexandr Morozov <lk4d4@docker.com>
Merge pull request #9606 from LK4D4/fix_race_in_test

Fix race in TestDaemonRestartWithVolumesRefs