Add "stress" tests to GitLab CI

Add "stress" tests to GitLab CI

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.