How to make mocha let my program write files?

I have a very strange behaviour (from my point of view):

I cannot write any file when I make my unit tests using mocha (I can create the files but nothing is written to them).

As an example I have this piece of code:

    console.log('The beginning');
    var fs = require('fs');

    var writableStream = fs.createWriteStream("./testOuputFile.txt");

    function callback(s){
        console.log('callback', s);
    }

    writableStream.on("error", function(err) {
        console.log("ERROR");
        done(err);
    });
    writableStream.on("close", function(ex) {
        console.log("CLOSED");
        done();
    });
    writableStream.on("finish", function(ex) {
        console.log("ENDED");
        done();
    });
    writableStream.on("open", function(fd) {
        console.log("OPENED:"+fd);
    });
    if(typeof callback === "undefined"){
        callback = function(s){console.log(s);};
    }
    var cbCalled = false;
    function done(err) {
        if (!cbCalled) {
            callback(err);
            cbCalled = true;
        }
    }
    writableStream.write('Something');
    writableStream.end();

    fs.writeFile('message.txt', 'Hello Node', function (err) {
        if (err) throw err;
        console.log('It\'s saved!');
    });

    console.log('The end');

Copying this code into a file named run.js and run 'node run.js' will output:

    The beginning
    The end
    OPENED:11
    ENDED
    callback undefined
    CLOSED
    It's saved!

I obtain two nice files containing some text: testOutputFile.txt and message.txt

Doing the same into a file named test/test.js and run 'mocha' will output:

    The beginning
    The end



        0 tests complete (0 ms)

And I obtain two empty files (zero bytes): testOutputFile.txt and message.txt

Is it normal ?

Is there something to tune in mocha to make this work ?