Replace invoking `is` with string

`is` is deprecated in `@vue/test-utils` 1.x

spec/frontend/vue_mr_widget/components/mr_widget_icon_spec.js

@@ -20,7 +20,7 @@ describe('MrWidgetIcon', () => {
   });
 
   it('renders icon and container', () => {
-    expect(wrapper.is('.circle-icon-container')).toBe(true);
+    expect(wrapper.element.className).toContain('circle-icon-container');
     expect(wrapper.find(GlIcon).props('name')).toEqual(TEST_ICON);
   });
 });

spec/frontend/vue_shared/components/actions_button_spec.js

 import { shallowMount } from '@vue/test-utils';
-import { GlDropdown, GlButton } from '@gitlab/ui';
+import { GlDropdown, GlDropdownDivider, GlButton } from '@gitlab/ui';
 import { createMockDirective, getBinding } from 'helpers/vue_mock_directive';
 import ActionsButton from '~/vue_shared/components/actions_button.vue';
 
@@ -53,7 +53,7 @@ describe('Actions button component', () => {
     findDropdown()
       .findAll('gl-dropdown-item-stub,gl-dropdown-divider-stub')
       .wrappers.map((x) => {
-        if (x.is('gl-dropdown-divider-stub')) {
+        if (x.is(GlDropdownDivider)) {
           return { type: 'divider' };
         }