[Java] crate-java-testing

[amotl]

Problem

Both patches reflecting upon software packages which can be used for testing CrateDB did not include presenting the crate-java-testing package for Java. It is well alive.

• Testcontainers for Java: Add showcase examples for different scenarios #54
• Testing: Demonstrate Python test layer implementations #280

Solution

Add a relevant snippet to cratedb-examples, and a corresponding documentation section to cratedb-guide, at https://cratedb.com/docs/guide/integrate/testing.html.