Unit testing this library does not give adequate confidence in the
working state of the overall system. The provided `systemtest` Makefile
target can be used to run a very broad system-level test using Jenkins
running in a local Docker context. It's not safe to run in CI (requires
bind mounting the Docker socket within the Jenkins container so that it
can actual perform builds) but it's useful for local development.

Change-Id: Ib9645afa993fa7d247321f3307de30bcab627c20

Establish a `org.wikimedia.integration` package for housing pipeline
related Groovy code, and a test of unit tests. Gradle configuration is
provided for running the tests either via Blubber/Docker (see
`.pipeline/blubber`) or directly (run `gradle test`).

Bug: T196940
Change-Id: I0a72200b9e24f71a706718a107e4941c0e772af8