Improve selection_summary_spec.js tests

This improves the tests of the selection_summary_spec.js by switching to
async/await and replacing the "ref" attribute with a "data-testid" one.

ee/app/assets/javascripts/security_dashboard/components/selection_summary.vue

@@ -103,7 +103,7 @@ export default {
 <template>
   <div class="card">
     <form class="card-body d-flex align-items-center" @submit.prevent="handleDismiss">
-      <span ref="dismiss-message">{{ message }}</span>
+      <span data-testid="dismiss-message">{{ message }}</span>
       <gl-form-select
         v-model="dismissalReason"
         class="mx-3 w-auto"

ee/spec/frontend/security_dashboard/components/selection_summary_spec.js

@@ -23,7 +23,7 @@ describe('Selection Summary component', () => {
   };
 
   const dismissButton = () => wrapper.find(GlButton);
-  const dismissMessage = () => wrapper.find({ ref: 'dismiss-message' });
+  const dismissMessage = () => wrapper.find('[data-testid="dismiss-message"]');
   const formSelect = () => wrapper.find(GlFormSelect);
   const createComponent = ({ props = {}, data = defaultData, mocks = defaultMocks } = {}) => {
     spyMutate = mocks.$apollo.mutate;
@@ -62,14 +62,15 @@ describe('Selection Summary component', () => {
       it('should have the button enabled if a vulnerability is selected and an option is selected', async () => {
         expect(wrapper.vm.dismissalReason).toBe(null);
         expect(wrapper.findAll('option')).toHaveLength(4);
-        formSelect()
+
+        const option = formSelect()
           .findAll('option')
-          .at(1)
-          .setSelected();
+          .at(1);
+        option.setSelected();
         formSelect().trigger('change');
-        await wrapper.vm.$nextTick();
 
-        expect(wrapper.vm.dismissalReason).toEqual(expect.any(String));
+        await wrapper.vm.$nextTick();
+        expect(wrapper.vm.dismissalReason).toEqual(option.attributes('value'));
         expect(dismissButton().attributes('disabled')).toBe(undefined);
       });
     });