Update Javascript tests

This only updates the JavaScript tests where we don't need to make changes to
the actual JavaScript too.

tests/javascripts/preventDuplicateFormSubmissions.test.js

@@ -19,7 +19,7 @@ describe('Prevent duplicate form submissions', () => {
     // set up DOM
     document.body.innerHTML = `
       <form action="/" method="post">
-        <button class="button" type="submit">Continue</button>
+        <button class="govuk-button" type="submit">Continue</button>
       </form>`;
 
     form = document.querySelector('form');

tests/javascripts/previewPane.test.js

@@ -73,11 +73,11 @@ describe('Preview pane', () => {
                   <input autocomplete="off" class="form-control form-control-1-1 " id="search" name="search" required="" rows="8" type="search" value="">
                 </div>
               </div>
-            </div> 
+            </div>
           </div>
         </div>
         <div class="page-footer">
-          <button type="submit" class="button">Preview</button>
+          <button type="submit" class="govuk-button">Preview</button>
         </div>
       </form>`;
 

tests/javascripts/stick-to-window-when-scrolling.test.js

@@ -91,7 +91,7 @@ describe("Stick to top/bottom of window when scrolling", () => {
                 </div>
               </div>
               <div class="page-footer">
-                <button type="submit" class="button">Continue</button>
+                <button type="submit" class="govuk-button">Continue</button>
               </div>
             </form>
           </main>
@@ -701,7 +701,7 @@ describe("Stick to top/bottom of window when scrolling", () => {
             </div>
             <div class="page-footer js-stick-at-bottom-when-scrolling">
               <form method="post" action="">
-                  <button type="submit" class="button">Send 1 email </button>
+                  <button type="submit" class="govuk-button">Send 1 email </button>
               </form>
             </div>
           </div>