moduleFor & friends: Support nested modules as provided by QUnit.module

[jeremy-w]

QUnit.module supports nested modules. Usage looks like:

QUnit.module("top-level module", function() {
  QUnit.test("a test in the top-level module", function(assert) {
    assert.ok(true, "yup");
  });

  QUnit.module("nested module", function() {
    QUnit.test("a test in the nested module", function(assert) {
      assert.ok(true, "yup");
    });
  });
});

I figured moduleFor might support this already, but it does not. When there's repeated setup context related to a scenario, it would be convenient to be able to nest modules to express this.

[rwjblue]

Please review and chime in on https://github.com/rwjblue/ember-qunit/issues/208.