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.

(cherry picked from commit 39305411e85fb4423b6b8248d4c69a9003cd16e8)
(cherry picked from commit 06f721fdc521423869e467d0f2416a9cdd3d947f)