Fix collapse.js aria-expanded behavior

* Remove aria-expanded from collapse.js target element

aria-expanded="true"/aria-expanded="false" only applies to the trigger,
not the element that is being expanded/collapsed.

* Tweak collapse.js accessibility section

...to make it clearer that the aria-expanded attribute always just goes
on the control.

* Fix collapse.js unit tests

- reword some of the text to make it clear we're checking behavior of
trigger/control
- move incorrect aria-expanded out of the <div>s and to the actual
trigger/control <a>s
- fix incorrect test assertion text output false -> true

js/src/collapse.js

@@ -162,7 +162,6 @@ const Collapse = (($) => {
         .addClass(ClassName.COLLAPSING)
 
       this._element.style[dimension] = 0
-      this._element.setAttribute('aria-expanded', true)
 
       if (this._triggerArray.length) {
         $(this._triggerArray)
@@ -223,8 +222,6 @@ const Collapse = (($) => {
         .removeClass(ClassName.COLLAPSE)
         .removeClass(ClassName.SHOW)
 
-      this._element.setAttribute('aria-expanded', false)
-
       if (this._triggerArray.length) {
         $(this._triggerArray)
           .addClass(ClassName.COLLAPSED)
@@ -300,7 +297,6 @@ const Collapse = (($) => {
     _addAriaAndCollapsedClass(element, triggerArray) {
       if (element) {
         const isOpen = $(element).hasClass(ClassName.SHOW)
-        element.setAttribute('aria-expanded', isOpen)
 
         if (triggerArray.length) {
           $(triggerArray)