try out testcontainers for a couple tests

[amohrland]

Trying out use of testcontainers for tests against Kafka and Schema Registry. Also, convert scalatest tests to munit.

[amohrland]

These run fine locally, but fail on Travis. My guess is that this is due to port conflicts between the still-running docker-compose that we use for the other tests.

[amohrland]

Also, amusingly, I needed to remove the examples build, to compile the project. For some reason, changes to tests in core caused RecordFormats to not be found in the examples. This will need to be figured out.