fix test memory leaks

ember-cli-memory-leak-detector identified 3 leaked instances of
PowerSelect after a full run of the test suite.
Converting 3 instances of onclick to {{on "click ...}} in test code
fixed the leaks that were found.

tests/integration/components/power-select/customization-with-components-test.js

@@ -197,11 +197,14 @@ module('Integration | Component | Ember Power Select (Customization using compon
   test('the `@beforeOptionsComponent` and `@afterOptionsComponent` receive the `@extra` hash', async function(assert) {
     assert.expect(1);
     this.owner.register('component:custom-before-options2', Component.extend({
-      layout: hbs`<button class="custom-before-options2-button" onclick={{@extra.passedAction}}>Do something</button>`
-    }));
-    this.owner.register('component:custom-after-options2', Component.extend({
-      layout: hbs`<button class="custom-after-options2-button" onclick={{@extra.passedAction}}>Do something</button>`
+      layout: hbs`<button class="custom-before-options2-button" {{on "click" @extra.passedAction}}>Do something</button>`
     }));
+    this.owner.register(
+      "component:custom-after-options2",
+      Component.extend({
+        layout: hbs`<button class="custom-after-options2-button" {{on "click" @extra.passedAction}}>Do something</button>`,
+      })
+    );
     let counter = 0;
     this.countries = countries;
     this.country = countries[1]; // Spain

tests/integration/components/power-select/general-behaviour-test.js

@@ -1192,7 +1192,7 @@ module('Integration | Component | Ember Power Select (General behavior)', functi
     };
 
     await render(hbs`
-      <button id="refresh-collection-btn" onclick={{action refreshCollection}}>Refresh collection</button>
+      <button id="refresh-collection-btn" {{on "click" (action refreshCollection)}}>Refresh collection</button>
       <br>
       <PowerSelect @options={{options}} @selected={{selected}} @onChange={{action (mut selected)}} as |name|>
         {{name}}