Adjust rabbitmq-basics test to work on rabbitmq 3.9

[yosifkit]

New RabbitMQ 3.9 build removes all env support (and thus our default config). Add a permissive config for the test.

Related to docker-library/rabbitmq#467

$ # after building 3.9-rc from docker-library/rabbitmq#467
$ ./test/run.sh -t rabbitmq-basics rabbitmq:3.9-rc-test rabbitmq:3.8
testing rabbitmq:3.9-rc-test
	'rabbitmq-basics' [1/1]....passed
testing rabbitmq:3.8
	'rabbitmq-basics' [1/1].....passed