I'm testing a collaborating unit, that passed the result of notesList module to buildPage module.

  before(function () {
    subject = SandboxedModule.require('../../router', {
        singleOnly: true,
        requires:   {
            './buildPage':    buildPage = sinon.stub(),
            './notesList':    notesList = sinon.stub()
        }
    });
});

//...

before("notesList returns an html fragment in a promise", function () {
    notesList.withArgs(sinon.match.string).returns(
        new Promise(function (resolve) {
            resolve("<pants>fragment</pants>");
        })
    );
});

Then to test if my router module passes correctly the results:

  context("responds to '', '/','/oracle','/oracle/','/oracle/...' routes", function(){
    before("called with ''", function () {
        subject({url: ""});
    });
    before("called with '/'", function () {
        subject({url: "/"});
    });
    // ...
    it("calls buildPage with the result of notesList", function(done){
        expect(buildPage.calledWith("<pants>fragment</pants>")).to.be.ok;
        done();
    });
});

At line #9 I added the ​done​ parameter to make it wait for the Promise to return.