Python DTest does not release CQL connections, which can cause containers to hang for 2 hours trying to connect to a server thats already gone

While debugging why upgrade tests were taking 2 hours we noticed that the test completed 2 hours early… yet the process did not exit and was logging cassandra.pool trying to connect to a cluster (but python dtest already cleaned it up)…

After investigating the code I saw that we create connections and store them in an array, but only 2 tests actually clean up!