Add testcontainers example

[binkley]

https://www.testcontainers.org/ is recommended for system/integration testing needing to bring up the full application, connecting to a disposable test database, etc.

These kinds of tests should be rare for the main code base which relies on unit tests or "slice" tests ala Spring Boot's approach, and so forth, but are important for higher levels of the test pyramid like user journey tests and such.

[binkley]

Use https://github.com/binkley/kotlin-spring-boot-hateoas-database for an example.