The time check in the checkMultiThread.js test is just comparing the millisecond value of the thread, it should be comparing the the total time each thread took to execute relative to the start time. This is the proposed change:

var func = function() {
    var start = new Date();
    db.runCommand({$eval: "sleep(10000);", nolock: true});
    return new Date();
};

assert.lt(
    a.returnData() - start, 15000, "A took more than 15s");