如何使用现有的xml testng套件添加参数线程数？

Question

我想在现有套件xml文件gradle 自定义任务中设置参数thread-count，类型为Test。我想设置useTestNG，然后使用 TestNG.class 进行测试

String sourceDir = "src"
String resourcesPath = sourceDir.concat("/test/resources")

sourceSets {
test {
    java {
        srcDirs = [sourceDir]
        }
    resources {
        srcDirs = [resourcesPath]
        }
    }
}

apply plugin: 'java'

dependencies {
compile project(':core')
compile group: 'org.testng', name: 'testng', version: '6.1.1'
}

task customTask(type: Test) {

    useTestNG {
        String threads = System.getenv("THREADS")
        String suiteName = System.getenv("SUITE")
        useDefaultListeners = true
        testLogging {
            events "passed", "skipped", "failed"
        }
     }

    //What I trying to do, but i have no acces to testng package

    TestNG tng = new TestNG();
    tng.setXmlSuites(suiteName);
    tng.setThreadCount(threads)
    tng.run(); 
}

那么我如何使用现有的xml testng套件添加参数线程数？我是否已创建TestNG实例，从现有文件设置套件并设置threadCount？我如何从exml xml文件创建套件对象？