This change enables cleaner separation between "actual tests" and "support classes for the tests".
For instance, `:src:core` might provide dummy samplers like `ThreadSleep` for testing purposes.
`ThreadSleep` would depend on `src/test/main` (since it would `extend AbstractSampler`),
and other modules (e.g. tests in `:src:protocols:http`) might need that `ThreadSleep` sampler.
It won't be practical to create `core-testkit` modules for every module,
so we move those reusable test classes to `testFixtures/...` source set.
See https://docs.gradle.org/current/userguide/java_testing.html#sec:java_test_fixtures