add test for sha-mismatch state component

spec/javascripts/vue_mr_widget/components/states/mr_widget_sha_mismatch_spec.js

+import Vue from 'vue';
+import shaMismatchComponent from '~/vue_merge_request_widget/components/states/mr_widget_sha_mismatch';
+
+describe('MRWidgetSHAMismatch', () => {
+  describe('template', () => {
+    const Component = Vue.extend(shaMismatchComponent);
+    const vm = new Component({
+      el: document.createElement('div'),
+    });
+    it('should have correct elements', () => {
+      expect(vm.$el.classList.contains('mr-widget-body')).toBeTruthy();
+      expect(vm.$el.querySelector('button').getAttribute('disabled')).toBeTruthy();
+      expect(vm.$el.innerText).toContain('The source branch HEAD has recently changed. Please reload the page and review the changes before merging.');
+    });
+  });
+});