Replace TestSourcesPlugin with Gradle test fixture support

[sbrannen]

Overview

We should consider replacing our custom TestSourcesPlugin with Gradle's official support for Test Fixtures.

Rationale

• Currently all test code is visible in all other (downstream) projects, and that makes it too easy to introduce unnecessary coupling.
  • For example, this made it more difficult to migrate to JUnit Jupiter.
• Having test fixture code in a dedicated source folder makes it readily apparent that the code is reused across the test suite.
• It will be much cleaner if projects explicitly declare that they rely on specific test fixtures of upstream projects.

Related Issues

• Reorganize Gradle build  #23282

[marcphilipp]

It will also make builds faster because test classes in downstream projects won't have to be recompiled and tests won't have to be executed if only a test class changes in an upstream project.