Add Visual Test for Snow Theme and add visual tests to PR execution (#5570)

e2e/helper/useSnowTheme.js

@@ -0,0 +1,30 @@
+/*****************************************************************************
+ * Open MCT, Copyright (c) 2014-2022, United States Government
+ * as represented by the Administrator of the National Aeronautics and Space
+ * Administration. All rights reserved.
+ *
+ * Open MCT is licensed under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * http://www.apache.org/licenses/LICENSE-2.0.
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ *
+ * Open MCT includes source code licensed under additional open source
+ * licenses. See the Open Source Licenses file (LICENSES.md) included with
+ * this source code distribution or the Licensing information page available
+ * at runtime from the About dialog for additional information.
+ *****************************************************************************/
+
+// This should be used to install the Snow theme for Open MCT. Espresso is the default
+// e.g.
+// await page.addInitScript({ path: path.join(__dirname, 'useSnowTheme.js') });
+
+document.addEventListener('DOMContentLoaded', () => {
+    const openmct = window.openmct;
+    openmct.install(openmct.plugins.Snow());
+});