tests(selectize): Add controllers for selectize

[karangattu]

The selectize test had been disabled/skipped for some time since it was using the same controllers as Input Select even though their implementation is drastically different. This PR adds a revamped controller for selectize instead of using the same one as select or the mixin SelectBase

Select DOM structure

<select class="shiny-input-select form-select shiny-bound-input" id="state">
   <optgroup label="East Coast">
      <option value="NY" selected="">New York</option>
      <option value="NJ">New Jersey</option>
      <option value="CT">Connecticut</option>
   </optgroup>
   <optgroup label="West Coast">
      <option value="WA">Washington</option>
      <option value="OR">Oregon</option>
      <option value="CA">California</option>
   </optgroup>
   <optgroup label="Midwest">
      <option value="MN">Minnesota</option>
      <option value="WI">Wisconsin</option>
      <option value="IA">Iowa</option>
   </optgroup>
</select>

Selectize DOM structure

   <select class="shiny-input-select selectized shiny-bound-input" id="state" multiple="multiple" tabindex="-1" style="display: none;">            
   </select>
   <div class="shiny-input-select selectize-control multi plugin-remove_button">
      <div class="selectize-input items not-full has-options"><input type="text" autocomplete="new-password" autofill="no" tabindex="" id="state-selectized" style="width: 4px; opacity: 1; position: relative; left: 0px;"></div>
      <div class="multi selectize-dropdown shiny-input-select plugin-remove_button" style="display: none; width: 100%; min-width: 300px; top: 36.5px; left: 0px; visibility: visible;">
         <div class="selectize-dropdown-content" tabindex="-1">
            <div class="optgroup" data-group="East Coast">
               <div class="optgroup-header">East Coast</div>
               <div class="option" data-selectable="" data-value="NY">New York</div>
               <div class="option" data-selectable="" data-value="NJ">New Jersey</div>
               <div class="option" data-selectable="" data-value="CT">Connecticut</div>
            </div>
            <div class="optgroup" data-group="West Coast">
               <div class="optgroup-header">West Coast</div>
               <div class="option" data-selectable="" data-value="WA">Washington</div>
               <div class="option" data-selectable="" data-value="OR">Oregon</div>
               <div class="option" data-selectable="" data-value="CA">California</div>
            </div>
            <div class="optgroup" data-group="Midwest">
               <div class="optgroup-header">Midwest</div>
               <div class="option" data-selectable="" data-value="MN">Minnesota</div>
               <div class="option" data-selectable="" data-value="WI">Wisconsin</div>
               <div class="option" data-selectable="" data-value="IA">Iowa</div>
            </div>
         </div>
      </div>
   </div>
   <script type="application/json" data-for="state" data-eval="[]">{"plugins": ["remove_button"]}</script>