Enforce a run time limit on unit test binaries

When a unit test binary hangs, the GitLab CI job in which it is run is stuck until its run time limit is exceeded. Furthermore, it is not trivial to determine which test(s) hung in a given GitLab CI job based on its log. To prevent these issues, enforce a run time limit on every binary executed by the lib/unit-test-driver.sh script. Use a timeout of 5 minutes for consistency with older BIND 9 branches, which employed Kyua for running unit tests. Report an exit code of 124 when the run time limit is exceeded for a unit test binary, for consistency with the "timeout" tool included in GNU coreutils.

Closes #2579 (closed)