The test is more or less random load test, in addition it creates and closes a lot of connections
which makes a not insufficiently tweaked buildbot machine by Foundation to give up

CURRENT_TEST: main.mdev-504
mysqltest: At line 70: query 'connect con3,localhost,root,,' failed: <Unknown> (2002): Can't connect to server on 'localhost' (10048)

The Winsock error hints that all ephemeral addresses are in use, this happens if too many sockets are in TIMED_WAIT state, and this happens when too many connections are created and closed very fast, which given 300 connects and disconnects is rather likely.

The author of the test confesses that this test is a mistake, and agreed with removal by (but on some reason she does not like to touch it herself)