TestMutableClass.swift

func testMutableClass() {
    let group = DispatchGroup()
    let obj = MutableClass()

    for _ in 0...1000 {
        group.enter()

        DispatchQueue.global().async {
            let sleepVal = arc4random() % 1000
            usleep(sleepVal)
            obj.update(newVal: 42)
            group.leave()
        }
    }

    let result = group.wait(timeout: DispatchTime.now() + 5)

    XCTAssert(result == .success)
}