[iot] fix: use fixtures for resource setup/teardown (#3428)

* [iot] fix: use fixtures for resource setup/teardown

fixes #2861
fixes #3323
fixes #2859
fixes #2860

* avoid topic name conflict
* refactored out the fixtures
* share the fixtures within two tests
* correct dependencies between fixtures
* use threading for cordinating between background and foreground
  threads.

Author: Takashi Matsuo