Fix bower error

By some reason ember-try is checking for bower looking at
scenario.bower, scenario.dependencies and scenario.devDependencies:
https://github.com/ember-cli/ember-try/blob/8fca8fc3aad863c5902a965287d728415a4a4663/lib/utils/dependency-manager-adapter-factory.js#L20

Fix the error by moving devDependencies into npm object which is correct
anyway, not sure how it worked before. Seems that peerDependencies also
should be nullified as otherwise there is an error:
```
ember-exam tried to import "ember-qunit" in "ember-exam/test-support/-private/ember-exam-qunit-test-loader.js" but the package was not resolvable from <path>/ember-exam
```

Author: Andrey Fel

tests/dummy/config/ember-try.js

@@ -6,11 +6,16 @@ const mergeWith = require('lodash.mergewith');
 
 function mochaScenario(scenario = {}) {
   return mergeWith({}, scenario, {
-    devDependencies: {
-      'chai-dom': '*',
-      'ember-cli-chai': '*',
-      'ember-mocha': '*',
-      'ember-qunit': null,
+    npm: {
+      devDependencies: {
+        'chai-dom': '*',
+        'ember-cli-chai': '*',
+        'ember-mocha': '*',
+        'ember-qunit': null,
+      },
+      peerDependencies: {
+        'ember-qunit': null,
+      }
     },
   });
 }