Updates blubber config to v3 and uses a builder command to load gradle
dependencies separately from running the tests.
Change-Id: I8c08412ec1b64fac16bbf7534e2df3bf74f4fe33
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