Hung unit tests are never terminated

When Kyua is used for running unit tests (which is the case for BIND versions older than 9.17), it enforces a default 5-minute run time limit on every binary it executes. This prevents hung unit tests from sticking around until the GitLab CI job run time limit is exceeded.

As use of Kyua was dropped in BIND 9.17, hung unit tests run for the main branch are never terminated until the whole GitLab CI job is torn down due its run time limit being exceeded. Furthermore, it is not easy to tell which unit test hung in a given GitLab CI job without post-processing its log. Example:

https://gitlab.isc.org/isc-projects/bind9/-/jobs/1562799

(In this case, it was lib/isc/tests/tlsdns_test that hung.)

The lib/unit-test-drivers.sh unit test driver should be extended to terminate uncooperative unit test binaries in a timely fashion.