java-test-fixtures should propagate testFixturesRuntimeOnly dependencies

[tbroyer]

Currently, dependencies declared in testFixturesRuntimeOnly configuration (with the java-test-fixtures plugin) aren't propagated to the testRuntimeClasspath (I haven't tested but I suppose that's also the case when importing the project's test fixtures in another project).

Expected Behavior

Dependencies declared as testFixturesRuntimeOnly are propagated to the runtime classpath of anything that uses the test fixtures library.

Current Behavior

Dependencies declared as testFixturesRuntimeOnly are only available in the project's testFixturesRuntimeClasspath (which almost nobody ever use I suppose).

Context

I have unit and functional tests (using separate source sets) and want to share a JUnit 5 ArgumentProvider between them, so I thought about using the java-test-fixtures. I'm declaring the junit-jupiter-api and junit-jupiter-params as testFixturesApi, and that works well. But declaring the junit-jupiter-engine as testFixturesRutimeOnly doesn't make it available at runtime to the test and functionalTest tasks (through in the testRuntimeClasspath and functionalTestRuntimeClasspath, respectively).

I'm writing a new plugin (to share dependency capabilities to detect conflicts in logging frameworks; see #11300) and started with the sample project from gradle init from Gradle 5.6.4, then I added the java-test-fixtures plugin.

As a workaround, because functionalTestRuntimeOnly extends testRuntimeOnly, I could declare junit-jupiter-engine as testRuntimeOnly; but I was actually thinking about removing that inheritance (extension) between the source sets / configurations, specifically because they ought to be inherited (as dependencies) through the test fixtures.
I could also declare it as testFixturesImplementation even though I won't actually use it from the test fixture's code.

There must be other use-cases where you'd want to add a runtime dependency to test fixtures. Also, the testFixturesRuntimeOnly and testFixturesRuntimeClasspath configurations currently exist and are properly provisioned (looking at gradle dependencies), but are never actually used. I also noticed that testFixturesApiElements and testFixturesRuntimeElements configurations exist, but they are empty.

[tbroyer]

(fwiw, I backed up due to #11094, and am now using nebula.integtest to wire up my functional tests' source set: the shared code is in the test source set, and available from the integTest source set; not ideal but it works)

[mcmah309]

Issue fixed by upgrading from Gradle 6.8 to Gradle 7.3.3