Ensure setup / teardown respect promises.

The prior code completely ignored any promises being returned, and as
such forced a bunch of "run loop sprinkles" throughout the code.

This brings back the ability to return promises from `beforeEach` /
`setup` and from `teardown`.

packages/internal-test-helpers/lib/ember-dev/setup-qunit.js

@@ -18,14 +18,16 @@ export default function setupQUnit(assertion, _qunitGlobal) {
       assertion.reset();
       assertion.inject();
 
-      originalSetup.apply(this, arguments);
+      return originalSetup.apply(this, arguments);
     };
 
     options.teardown = function() {
-      originalTeardown.apply(this, arguments);
+      let result = originalTeardown.apply(this, arguments);
 
       assertion.assert();
       assertion.restore();
+
+      return result;
     };
 
     return originalModule(name, options);

packages/internal-test-helpers/lib/module-for.js

@@ -13,7 +13,7 @@ export default function moduleFor(description, TestClass, ...mixins) {
     },
 
     teardown() {
-      context.teardown();
+      return context.teardown();
     }
   });