Add aria-pressed to single toggle buttons

also includes tiny fix/clarification to two existing unit tests for the
.active class, adding data-toggle=“button” explicitly to the tested
buttons

Closes #14819.

js/button.js

@@ -60,6 +60,8 @@
         else $parent.find('.active').removeClass('active')
       }
       if (changed) $input.prop('checked', !this.$element.hasClass('active')).trigger('change')
+    } else {
+      this.$element.attr('aria-pressed', !this.$element.hasClass('active'))
     }
 
     if (changed) this.$element.toggleClass('active')