I wanted to include one directory form an svn repo.
With help of stackoverflow I found the solution.

"require-dev": {
    "wordpress/phpunit": "*"
},
"repositories": [
    {
        "type": "package",
        "package": {
            "name": "wordpress/phpunit",
            "version": "1.0.0",
            "source": {
                "url": "https://develop.svn.wordpress.org",
                "type": "svn",
                "reference": "trunk/tests/phpunit/includes/@43534"
            }
        }
    }
]

There is a downside, it won’t update. Unless you up the version inside the repositories block.