Update Failure tests

* Remove the impossible test, Failure8.  We should not have any test
  cases that fail by design.
* Add tests for leaving range of access point.
New test: DGI loses one device during convergence

But convergence continues. (Ensures groups don't reform.)
Revert "No point in calling Stop here..."

This reverts commit 212016e5e8733c2c708de00c9cf061660b103e3f.

Could be a problem if we synchronous read forever
Fix a potential hang Aaron discovered

This can't happen in our current code, but it will if we merge the
serial work.