How to copy phpunit test xml file of a module under dev/tests/unit/ with composer installation in Magento 2?

I wrote a module including phpunit test xml file. I want to automatically copy that xml file under dev/tests/unit/ after composer update.

The composer.json of the module looks like

{
    "name": "myvendor/mymodule",
    "description": "myvendor mymodule",
    "require": {
        "php": "~5.5.22|~5.6.0|~7.0.2|7.0.4|~7.0.6"
    },    
    "type": "magento2-module",
    "version": "1.0.0",
    "license": [
        "proprietary"
    ],
    "autoload": {
        "files": [ "registration.php" ],
        "psr-4": {
            "MyVendor\\MyModule\\": ""
        }
    },
    "extra": {
        "map": [
            [
                "mymodule-phpunit.xml",
                "dev/tests/unit/mymodule-phpunit.xml"
            ]
        ]
    }
}

But the node extra/map does not work.