Sample of using Sinon to stub modules in Dojo AMD.

myservicespec.js

define([
  'esri/tasks/FindTask',
  'dojo/promise/Promise',
  'widgets/search/searchservice'
], function(
  FindTask,
  Promise,
  Widget) {

    return describe('widgets/search/searchservice', function() {


      var service,
      stubFind,
      options = {
        settings: {
          params: {},
          findUrl: 'testpath'
        }
      };

      beforeEach(function() {
        service = new Widget(options);
        stubFind = sinon.stub(FindTask.prototype, 'execute');
        stubFind.returns({
          then: function() {
            return 999;
          }
        });
      });

      afterEach(function() {
        stubFind.restore();
      });

      it(
        'is a valid object',
        function() {
          expect(service).to.be.ok();
          expect(service.options).to.be.ok();
          expect(service.options.settings).to.eql(options.settings);
        }
      );

      describe('#find', function() {
        it(
          'has its own service',
          function() {
            expect(service._task).to.be.ok();
          }
        );

        it(
          'returns a dojo/promise/Promise',
          function() {
            expect(service.find('999')).to.be.a(Promise);
          }
        );

        it(
          'calls a find task with its FindParameters',
          function() {
            service.find('999');
            expect(stubFind.called).to.be.ok();
            expect(stubFind.calledWith(service._findParams)).to.be.ok();
          }
        );

        it(
          'searches for given text',
          function() {
            var searchText = 'test string';
            service.find(searchText);
            expect(service._findParams.searchText).to.eql(searchText);
          }
        );
      });
    });

  });