Add tests for Gradle incremental compilation

[eamonnmcmanus]

We broke Gradle incremental compilation in AutoValue 1.8 and AutoFactory 1.0, and furthermore I had to make 3 attempts to fix it before succeeding. The problem is that we don't use Gradle internally and it isn't part of our tests. We should add a test that ensures that it doesn't break again.

@tbroyer wrote:

You could have a look at https://github.com/tbroyer/gradle-incap-helper/tree/v0.3/integTest/src/test/kotlin/net/ltgt/gradle/incap/integTest, it uses Gradle Test Kit (so has to be run from a Gradle build) to drive another Gradle build and check its output (this is based on log messages, because it's not that easy to do it otherwise: tbroyer/gradle-incap-helper#2)
Because this is testing net.ltgt.gradle.incap:incap-processor itself, the test code is itself an annotation processor; it would likely be simpler for the Auto projects.

Our tests are run from Maven, not Gradle, so it might be a bit trickier. We might need to launch one Gradle instance to have it run the test against a second Gradle instance.

[tbroyer]

The Dagger build deploys the artifacts to the local repository to be able to easily load them from the Gradle builds (run separately from the GitHub Actions workflow).

(they don't test incremental processing AFAICT, only that everything works correctly, which includes the way they generate the POMs and JARs from the Bazel build)

[eamonnmcmanus]

@tbroyer how does #1076 look to you?