Moved environment_external_url_spec to jest test

Moved file from spec/javascripts to spec/frontend.
Updated tests from karma testing to jest testing.

spec/frontend/environments/environment_external_url_spec.js

+import { shallowMount } from '@vue/test-utils';
+import ExternalUrlComp from '~/environments/components/environment_external_url.vue';
+
+describe('External URL Component', () => {
+  let wrapper;
+  const externalUrl = 'https://gitlab.com';
+
+  beforeEach(() => {
+    wrapper = shallowMount(ExternalUrlComp, { propsData: { externalUrl } });
+  });
+
+  it('should link to the provided externalUrl prop', () => {
+    expect(wrapper.attributes('href')).toEqual(externalUrl);
+    expect(wrapper.find('a').exists()).toBe(true);
+  });
+});

spec/javascripts/environments/environment_external_url_spec.js

-import Vue from 'vue';
-import externalUrlComp from '~/environments/components/environment_external_url.vue';
-
-describe('External URL Component', () => {
-  let ExternalUrlComponent;
-
-  beforeEach(() => {
-    ExternalUrlComponent = Vue.extend(externalUrlComp);
-  });
-
-  it('should link to the provided externalUrl prop', () => {
-    const externalURL = 'https://gitlab.com';
-    const component = new ExternalUrlComponent({
-      propsData: {
-        externalUrl: externalURL,
-      },
-    }).$mount();
-
-    expect(component.$el.getAttribute('href')).toEqual(externalURL);
-    expect(component.$el.querySelector('fa-external-link')).toBeDefined();
-  });
-});