Rewrite stress test to pytest

Rewrite stress test to pytest

The shell version of the test was completed only after all DNS zone
updates were sent, even if the BIND server crashed while processing
them, leading to prolonged execution and potential hang in the CI
environment. The Python rewrite of the test ensures that DNS update
tasks finish within five minutes of starting, irrespective of a BIND
crash possibility or DNS zone updates not finishing in time.