Avoid issues with fallback registry caching.

The way we are hooking into the application instance registry ultimately means
that we leak "factories". Once the first test resolves a factory for a
given name, **all other tests** will use that factory (even if
`setResolverRegistry` were called again with a different value!1!1!1!1).

This works around the immediate issue by always using the "current"
assert object instead of the closure scoped one because all of the tests
will use the same factory, and using `assert` from the first test will
cause many issues for subsequent tests.

In the long run, we need to fix this in a different way. Most likely
that means changing the way setResolverRegistry works to do something
like:

* hooking into the shared application instance
* calling unregister and reregister for each key in the pojo provided

Alternatively, we could recreate the registry instance itself (therefore
flushing any caches). I think either would work, but am not certain how
hard to implement...

tests/unit/setup-application-context-test.js

@@ -33,7 +33,7 @@ module('setupApplicationContext', function(hooks) {
     return;
   }
 
-  hooks.beforeEach(async function(assert) {
+  hooks.beforeEach(async function() {
     setResolverRegistry({
       'router:main': Router,
       'template:application': hbs`
@@ -57,10 +57,10 @@ module('setupApplicationContext', function(hooks) {
       }),
       'route:slow': Route.extend({
         model() {
-          assert.step('model start');
+          QUnit.config.current.assert.step('model start');
           return new Promise(resolve => {
             setTimeout(() => {
-              assert.step('model resolving');
+              QUnit.config.current.assert.step('model resolving');
               resolve();
             }, 50);
           });