Add support for Integration Testing using Testcontainers

[jxblum]

From @dturanski:

geode = new GenericContainer<>("apachegeode/geode:1.12.0");
geode.withCommand("tail","-f","/dev/null").start();

[jxblum]

Take @dturanski's example into consideration:

https://github.com/dturanski/geode-sandbox

[dturanski]

This is where I ended up https://github.com/spring-cloud/stream-applications/tree/master/functions/common/function-test-support/src/main/java/org/springframework/cloud/fn/test/support/geode . The only way I could get locator and cache server on non-default port is to ensure they are the same port in container and on the client. Exposing these ports using a custom Dockerfile with build args is the only thing that works for me.