Junit extension ought to respect the TEST_CLUSTER_EXECUTION_MODE environment variable

[k-wall]

As a user of the Junit5 annotation, I can use a @BrokerCluster to inject an Kafka cluster into my integration test. If I am opinionated about whether the test runs under a container or in-VM, I can express that using the type of the annotated field/formal parameter (InVMKafkaCluster|TestcontainersKafkaCluster). Sometimes the author of the test has no preference, so use they use KafkaCluster.

When the tests are run, KafkaClusterProvisioningStrategy makes a decision about whether the test case should use an in-VM or containers strategy, based on a number of factors. It is used to decide whether @BrokerCluster KafkaCluster takes the in-VM or container route. Currently it always decides in-VM owing to the estimatedProvisioningTimeMs.

As I user I would value the ability to influence the decision from the outside, possible via an environment variable. We already have TEST_CLUSTER_EXECUTION_MODE defined which is respected by the parts of the API.

[k-wall]

In addition, currently when using the extension, there's no mechanism to override the native images. There are env variables KAFKA_IMAGE_REPO, ZOOKEEPER_IMAGE_REPO but these are applied by the KafkaClusterFactory. The factory isn't used by the strategy.