Bootstrap acordeón con flechas

Solución:

El problema está en el espacio entre los selectores:

.panel-heading [data-toggle="collapse"]:after
              ^------- // remove this space to make this selector work

Ahora está seleccionando todos los elementos que tienen data-toggle atributo que son descendientes de .panel-heading. Debería ser:

.panel-heading[data-toggle="collapse"]:after

.panel-heading {
  position: relative;
}
.panel-heading[data-toggle="collapse"]:after {
  font-family: 'Glyphicons Halflings';
  content: "e072"; /* "play" icon */
  position: absolute;
  color: #b0c5d8;
  font-size: 18px;
  line-height: 22px;
  right: 20px;
  top: calc(50% - 10px);

  /* rotate "play" icon from > (right arrow) to down arrow */
  -webkit-transform: rotate(-90deg);
  -moz-transform:    rotate(-90deg);
  -ms-transform:     rotate(-90deg);
  -o-transform:      rotate(-90deg);
  transform:         rotate(-90deg);
}
.panel-heading[data-toggle="collapse"].collapsed:after {
  /* rotate "play" icon from > (right arrow) to ^ (up arrow) */
  -webkit-transform: rotate(90deg);
  -moz-transform:    rotate(90deg);
  -ms-transform:     rotate(90deg);
  -o-transform:      rotate(90deg);
  transform:         rotate(90deg);
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/js/bootstrap.min.js"></script>
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css" rel="stylesheet"/>
<!-- Accordion START -->
      <div class="panel-group" id="accordion">
        <div class="panel panel-default">
          <div class="panel-heading accordion-toggle collapsed" data-toggle="collapse" data-parent="#accordion" data-target="#collapseOne">
            <h4 class="panel-title">Test1</h4>

          </div>
          <div id="collapseOne" class="panel-collapse collapse">
            <div class="panel-body">
              <p>sadfsadfsdaf sadf</p>
            </div>
          </div>
        </div>
        <div class="panel panel-default">
          <div class="panel-heading accordion-toggle collapsed" data-toggle="collapse" data-parent="#accordion" data-target="#collapseTwo">
            <h4 class="panel-title">Test2</h4>

          </div>
          <div id="collapseTwo" class="panel-collapse collapse">
            <div class="panel-body">
<p>
Bla bla bla
</p>
            </div>
          </div>
        </div>
      </div>
      <!-- Accordion END -->

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