Remove exception method from QUnit test adapter.

As of Ember 2.17 the `Ember.Test.adapter` is not required to have an
`exception` method, and the method essentially has no purpose. The
default implementation (in the `Ember.Test.QUnitAdapter`) is to
re-throw, but that is _exactly_ what happens by default.

This removes "yet another magical testing thing", and relies on both
Ember and QUnit's default behaviors to ensure that tests properly fail
when unhandled RSVP rejections occur.

addon-test-support/ember-qunit/adapter.js

@@ -1,5 +1,6 @@
 import Ember from 'ember';
 import QUnit from 'qunit';
+import hasEmberVersion from '@ember/test-helpers/has-ember-version';
 
 function unhandledRejectionAssertion(current, error) {
   let message, source;
@@ -24,7 +25,7 @@ function unhandledRejectionAssertion(current, error) {
   });
 }
 
-export default Ember.Test.Adapter.extend({
+let Adapter = Ember.Test.Adapter.extend({
   init() {
     this.doneCallbacks = [];
   },
@@ -40,8 +41,18 @@ export default Ember.Test.Adapter.extend({
       done();
     }
   },
-
-  exception(error) {
-    unhandledRejectionAssertion(QUnit.config.current, error);
-  },
 });
+
+// Ember 2.17 and higher do not require the test adapter to have an `exception`
+// method When `exception` is not present, the unhandled rejection is
+// automatically re-thrown and will therefore hit QUnit's own global error
+// handler (therefore appropriately causing test failure)
+if (!hasEmberVersion(2, 17)) {
+  Adapter = Adapter.extend({
+    exception(error) {
+      unhandledRejectionAssertion(QUnit.config.current, error);
+    },
+  });
+}
+
+export default Adapter;