JUnit Engine should be managed

[melix]

Feature description

There's an inconsistency in how we handle the test frameworks. With kotest, we can use:

    testImplementation(mnTest.micronaut.test.kotest5)
    testRuntimeOnly(mnTest.kotest.runner.junit5.jvm)

To get the JUnit5 runtime dependency. But we have no such thing for JUnit, which forces to declare a dependency:

    testImplementation(mnTest.micronaut.test.junit5)
    testImplementation("org.junit.jupiter:junit-jupiter-engine:5.9.2")

Note that this is in a test-suite project, not a user project which would apply the Micronaut Gradle plugin, but still, for consistency and users wanting to avoid our plugins, there should be a managed version of JUnit.