Full testcov for <security-token-pin> Polymer elem

Add a bunch of unit tests for the <security-token-pin> Polymer element.

Bug: 1033936
Change-Id: I9b9545815c45c46074137604d71e0ddb72a2a081
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2116194Reviewed-by: Denis Kuznetsov [CET] <antrim@chromium.org>
Commit-Queue: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753248}

chrome/browser/resources/chromeos/login/security_token_pin.html

@@ -37,13 +37,14 @@
             <div id="errorContainer" role="alert" problem
                 invisible$="[[!isLabelVisible_(parameters, userEdited_)]]">
               <iron-icon id="errorIcon" icon="cr:error-outline"></iron-icon>
-              [[getLabel_(locale, parameters, errorLabelId_, userEdited_)]]
+              <span id="error">[[getLabel_(locale, parameters, errorLabelId_,
+                                           userEdited_)]]</span>
             </div>
           </pin-keyboard>
         </div>
       </div>
       <div slot="bottom-buttons" class="layout horizontal justified">
-        <oobe-back-button on-tap="onBackClicked_"></oobe-back-button>
+        <oobe-back-button id="back" on-tap="onBackClicked_"></oobe-back-button>
         <oobe-next-button id="submit" on-tap="onSubmit_"
             disabled="[[!canSubmit_]]"></oobe-next-button>
       </div>