Stop task runner before reaper in test teardown

[jy5275]

This PR solves issue#747.

To reproduce the that locally:

• In internals/overlord/servstate/manager_test.go change four plans to period: 1ms
• Then run while go test -race ./internals/overlord/servstate -count=10 -p 16 > /tmp/err; do date; done

You will probably see current master branch runs into panic within 3 loop rounds.

The reason is that: in internals/overlord/servstate/manager_test.go, TearDownTest calls reaper.Stop() without first stopping the task runner. This may cause a race condition where some tasks still fire goroutines and calling reaper.StartCommand, which panics if the reaper has been stopped.

Fix this by calling s.runner.Stop() before reaper.Stop().

I ran the issue-reproducing commands again for about 20+ loop rounds without panic locally.