Test jars should export only tests, and only be exported for select modules

Today, we have test-jars that are used as libraries for testing. That is not what "test jar" means, and dependency management actually does not work correctly for this. It is OK to depend on a test jar in order to run the tests therein, and not really OK to depend on one for another reason.

This ticket is a bucket ticket for fixes to this situation.

Imported from Jira [BEAM-3573](https://issues.apache.org/jira/browse/BEAM-3573). Original Jira may contain additional context.
Reported by: kenn.

Start by reviewing this bucket ticket and the imported BEAM-3573 Jira issue for the missing module-level context. Identify the test-jar exports and dependency-management behavior covered by the related fixes; done means test jars expose only tests and are produced only for selected modules.

Written by the indexing model from the issue text.