Add support for checking/generating API of testFixtures

[remcomokveld]

Kotlin JVM libraries support defining test fixtures using the java-test-fixtures (https://docs.gradle.org/current/userguide/java_testing.html#sec:java_test_fixtures) gradle plugin. These test fixtures can be published and should thus be considered part of the public API.

It would be nice if the binary-compatbility-validator would therefore generate a separate *.api file with the API of the test fixtures

[aSemy]

I've implemented test-fixtures API generation in my fork of BCV - if you'd like to give it a go then I'd appreciate the feedback

https://github.com/adamko-dev/kotlin-binary-compatibility-validator-mu

[tso]

I believe #116 would solve this. Is that correct @qwwdfsad and could you do a release if true? 🙏

[tso]

Update: Can confirm that this is solved with version 0.13.1 by configuring like so:

apiValidation {
  additionalSourceSets.addAll(listOf("testFixtures"))
}