Make loading spinner Pajamas compliant

Makes loading spinner Pajamas-compliant in the
merge-conflict-resolver-app component.

This actually fixes a visual bug as the spinner currently doesn't show
up at all.

Changelog: fixed

app/assets/javascripts/merge_conflicts/merge_conflict_resolver_app.vue

 <script>
-import { GlSprintf, GlButton, GlButtonGroup } from '@gitlab/ui';
+import { GlSprintf, GlButton, GlButtonGroup, GlLoadingIcon } from '@gitlab/ui';
 import { mapGetters, mapState, mapActions } from 'vuex';
 import { __ } from '~/locale';
 import FileIcon from '~/vue_shared/components/file_icon.vue';
@@ -23,6 +23,7 @@ export default {
     GlButton,
     GlButtonGroup,
     GlSprintf,
+    GlLoadingIcon,
     FileIcon,
     DiffFileEditor,
     InlineConflictLines,
@@ -72,9 +73,7 @@ export default {
 </script>
 <template>
   <div id="conflicts">
-    <div v-if="isLoading" class="loading">
-      <div class="spinner spinner-md"></div>
-    </div>
+    <gl-loading-icon v-if="isLoading" size="md" data-testid="loading-spinner" />
     <div v-if="hasError" class="nothing-here-block">
       {{ conflictsData.errorMessage }}
     </div>

spec/frontend/merge_conflicts/components/merge_conflict_resolver_app_spec.js

@@ -41,6 +41,7 @@ describe('Merge Conflict Resolver App', () => {
     wrapper.destroy();
   });
 
+  const findLoadingSpinner = () => wrapper.find('[data-testid="loading-spinner"]');
   const findConflictsCount = () => wrapper.find('[data-testid="conflicts-count"]');
   const findFiles = () => wrapper.findAll('[data-testid="files"]');
   const findFileHeader = (w = wrapper) => w.find('[data-testid="file-name"]');
@@ -60,6 +61,19 @@ describe('Merge Conflict Resolver App', () => {
     expect(title.text().trim()).toBe('Showing 3 conflicts between test-conflicts and main');
   });
 
+  it('shows a loading spinner while loading', () => {
+    store.commit('SET_LOADING_STATE', true);
+    mountComponent();
+
+    expect(findLoadingSpinner().exists()).toBe(true);
+  });
+
+  it('does not show a loading spinner once loaded', () => {
+    mountComponent();
+
+    expect(findLoadingSpinner().exists()).toBe(false);
+  });
+
   describe('files', () => {
     it('shows one file area for each file', () => {
       mountComponent();