Fix unit test and vue component

* Import GlButton and update unit tests

ee/app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_policy_violation.vue

 <script>
-import statusIcon from '~/vue_merge_request_widget/components/mr_widget_status_icon.vue';
+import { GlButton } from '@gitlab/ui';
+import StatusIcon from '~/vue_merge_request_widget/components/mr_widget_status_icon.vue';
 
 export default {
   name: 'MRWidgetPolicyViolation',
   components: {
-    statusIcon,
+    GlButton,
+    StatusIcon,
   },
 };
 </script>

ee/spec/frontend/vue_mr_widget/components/states/mr_widget_policy_violation_spec.js

 import { shallowMount } from '@vue/test-utils';
+import { GlButton } from '@gitlab/ui';
 import MrWidgetPolicyViolation from 'ee/vue_merge_request_widget/components/states/mr_widget_policy_violation.vue';
 
 describe('EE MrWidgetPolicyViolation', () => {
   let wrapper;
 
-  const findButton = () => wrapper.find('button');
+  const findButton = () => wrapper.find(GlButton);
 
   const createComponent = () => {
     wrapper = shallowMount(MrWidgetPolicyViolation, {});
@@ -21,7 +22,7 @@ describe('EE MrWidgetPolicyViolation', () => {
 
   it('shows the disabled merge button', () => {
     expect(wrapper.text()).toContain('Merge');
-    expect(findButton().attributes().disabled).toBe('disabled');
+    expect(findButton().attributes().disabled).toBe('true');
   });
 
   it('shows the disabled reason', () => {