Revert "runner_test: worked around timing issues causing test to hang when using the same MessageLoop"

This reverts commit be72f6eb1e943c1c1ab40533bdff90690a8c9afe.
runner_test: ensure that the Runner are properly removed from the message loop before destruction
runner: do not start the MessageLoop passed as parameter as it may have already been started before
runner: do not start the MessageLoop passed as parameter as it may have already been started before