function whilst(test, iteratee, callback) {
    callback = onlyOnce(callback || noop);
    var _iteratee = wrapAsync(iteratee);
    if (!test()) return callback(null);
    var next = function(err/*, ...args*/) {
        if (err) return callback(err);
        if (test()) return _iteratee(next);
        var args = slice(arguments, 1);
        callback.apply(null, [null].concat(args));
    };
    _iteratee(next);
}
Repeatedly call `iteratee`, while `test` returns `true`. Calls `callback` when
stopped, or an error occurs.

@name whilst
@static
@memberOf module:ControlFlow
@method
@category Control Flow
@param {Function} test - synchronous truth test to perform before each
execution of `iteratee`. Invoked with ().
@param {AsyncFunction} iteratee - An async function which is called each time
`test` passes. Invoked with (callback).
@param {Function} [callback] - A callback which is called after the test
function has failed and repeated execution of `iteratee` has stopped. `callback`
will be passed an error and any arguments passed to the final `iteratee`'s
callback. Invoked with (err, [results]);
@returns undefined
@example

var count = 0;
async.whilst(
    function() { return count < 5; },
    function(callback) {
        count++;
        setTimeout(function() {
            callback(null, count);
        }, 1000);
    },
    function (err, n) {
        // 5 seconds have passed, n = 5
    }
);