FIX: Prevent double slashes in Ember templates paths (PR #12630)

Follow-up to FEATURE: Introduce theme/component QUnit tests by OsamaSayegh · Pull Request #12517 · discourse/discourse · GitHub

Some themes add a leading slash to the names of the Ember templates they define/override, e.g.:

<script type='text/x-handlebars' data-template-name='/connectors/above-site-header/brand-header'>

So we need to add a check here to not break themes that do this.