Stop using strings for VTU stubs

@vuu/test-utils 1.x deprecated using strings for stubs
This MR makes fixes such deprecations

ee/spec/frontend/clusters/components/environments_spec.js

@@ -42,7 +42,7 @@ describe('Environments', () => {
     beforeAll(() => {
       wrapper = mount(Environments, {
         propsData: { ...propsData, environments },
-        stubs: { deploymentInstance: '<div class="js-deployment-instance"></div>' },
+        stubs: { deploymentInstance: { template: '<div class="js-deployment-instance"></div>' } },
       });
 
       table = wrapper.find(GlTable);

ee/spec/frontend/epic/components/epic_form_spec.js

@@ -27,7 +27,7 @@ describe('ee/epic/components/epic_form.vue', () => {
       },
       stubs: {
         ApolloMutation,
-        MarkdownField: '<div><slot name="textarea"></slot></div>',
+        MarkdownField: { template: '<div><slot name="textarea"></slot></div>' },
       },
       mocks: {
         $apollo: {

ee/spec/frontend/iterations/components/iteration_form_spec.js

@@ -37,7 +37,7 @@ describe('Iteration Form', () => {
       propsData: props,
       stubs: {
         ApolloMutation,
-        MarkdownField: '<div><slot name="textarea"></slot></div>',
+        MarkdownField: { template: '<div><slot name="textarea"></slot></div>' },
       },
       mocks: {
         $apollo: {

spec/frontend/alert_management/components/alert_details_spec.js

@@ -233,7 +233,7 @@ describe('AlertDetails', () => {
 
     describe('header', () => {
       const findHeader = () => wrapper.find('[data-testid="alert-header"]');
-      const stubs = { TimeAgoTooltip: '<span>now</span>' };
+      const stubs = { TimeAgoTooltip: { template: '<span>now</span>' } };
 
       describe('individual header fields', () => {
         describe.each`

spec/frontend/blob/suggest_gitlab_ci_yml/components/popover_spec.js

@@ -31,7 +31,7 @@ describe('Suggest gitlab-ci.yml Popover', () => {
         humanAccess,
       },
       stubs: {
-        'gl-popover': '<div><slot name="title"></slot><slot></slot></div>',
+        'gl-popover': { template: '<div><slot name="title"></slot><slot></slot></div>' },
       },
     });
   }

spec/frontend/monitoring/components/embeds/embed_group_spec.js

@@ -71,14 +71,14 @@ describe('Embed Group', () => {
 
     it('is expanded by default', () => {
       metricsWithDataGetter.mockReturnValue([1]);
-      mountComponent({ shallow: false, stubs: { MetricEmbed: '<div />' } });
+      mountComponent({ shallow: false, stubs: { MetricEmbed: true } });
 
       expect(wrapper.find('.card-body').classes()).not.toContain('d-none');
     });
 
     it('collapses when clicked', done => {
       metricsWithDataGetter.mockReturnValue([1]);
-      mountComponent({ shallow: false, stubs: { MetricEmbed: '<div />' } });
+      mountComponent({ shallow: false, stubs: { MetricEmbed: true } });
 
       wrapper.find(GlButton).trigger('click');
 
@@ -148,14 +148,14 @@ describe('Embed Group', () => {
   describe('button text', () => {
     it('has a singular label when there is one embed', () => {
       metricsWithDataGetter.mockReturnValue([1]);
-      mountComponent({ shallow: false, stubs: { MetricEmbed: '<div />' } });
+      mountComponent({ shallow: false, stubs: { MetricEmbed: true } });
 
       expect(wrapper.find(GlButton).text()).toBe('Hide chart');
     });
 
     it('has a plural label when there are multiple embeds', () => {
       metricsWithDataGetter.mockReturnValue([2]);
-      mountComponent({ shallow: false, stubs: { MetricEmbed: '<div />' } });
+      mountComponent({ shallow: false, stubs: { MetricEmbed: true } });
 
       expect(wrapper.find(GlButton).text()).toBe('Hide charts');
     });