Run "stress" tests for scheduled pipelines and pipelines created for
tags.  These tests were previously only performed manually (as part of
pre-release testing of each new BIND version).  Their purpose is to
detect memory leaks and potential performance issues.

As the run time of each "stress" test itself is set to 1 hour, set the
GitLab CI job timeout to 2 hours in order to account for the extra time
needed to set the test up and gather its results.