fix netmgr setup/teardown issues; setup and teardown only once in timer_test

fix netmgr setup/teardown issues; setup and teardown only once in timer_test

- the netmgr was not correctly being specified when creating the task
  manager, and was cleaned up in the wrong order when shutting down.
- on freebsd, timer_test appears to be prone to failure if the
  netmgr is set up and torn down before and after ever test case, but
  less so if it's only set up once at the beginning and once at the
  end.