Main-jar & Test-jar from regular code

testJar.gradle

// PROJECT A
val testsConfig by configurations.creating {
    extendsFrom(configurations["testImplementation"])
}
dependencies {
    testsConfig(sourceSets.getByName("test").output)
}
// PROJECT B
testImplementation(project(":projectA", "testsConfig"))

// OR: for maven dependencies
val testsJar by tasks.creating(Jar::class) {
    archiveClassifier.set("tests")
    from(sourceSets.getByName("test").output)
} // similar JAR definitions for sources and testsSources
publishing {
    publications {
        create<MavenPublication>("maven") {
            from(components["java"])
            artifact(testsJar)
        }
    }
}