This is a particular manifestation of IMPALA-3947. test_ddl_stress assigns its workload as "targeted-stress", which makes it not subject to buildall.sh "exhaustive" tests, which only apply to the workload "functional-query". This means any environment or commandline options to "run exhaustive tests" still won't cause test_ddl_stress.py to run.
This is a problem if people assumed test_ddl_stress.py had been running via buildall.sh this entire time. It's less of a problem if this is a known thing and people are very self-conscious about running test_ddl_stress.py locally.