sourceSets {
    test {
        java.srcDir file('src/main/java')
        resources.srcDir file('src/main/resources')
    }
}

sourceSets {
    test {
        java.srcDir 'src/main/java'
        resources.srcDir 'src/main/resources'
    }
}

sourceSets {
    test {
        java.srcDir file('src/main/java/tests') // tests shall contain your tests
        resources.srcDir file('src/main/resources')
        output.resourcesDir = "build/classes/main"
    }
}