Improve performance of image building plugin integration tests

[scottfrederick]

The Gradle plugin BootBuildImageIntegrationTests and Maven plugin BuildImageTests are designed to test the integration between the build plugins and a CNB builder. These tests take a lot of time to run because they use the default Paketo builder to build images and then verify that the images can be run. We'd like to reduce the time it takes to run these tests by using a test harness in place of the Paketo builder and do the full Paketo integration testing less frequently as described in #25824.

[philwebb]

Reopening to see if we can fix the Docker rate limiting issues and to also consider if we should move src/spring-boot-builder