Fix dropdown unit test (#29037)

swap jQuery's `trigger(...)` with the more verbose native `dispatchEvent(...)`, as the former may not always behave/bubble correctly (observed while trying to write unit tests for keyboard handling of ARIA tab navigation), which may lead to this test passing even though it fails in real usage.

js/tests/unit/dropdown.js

@@ -88,10 +88,11 @@ $(function () {
     $(dropdownHTML).appendTo('#qunit-fixture')
     var $dropdown = $('#qunit-fixture').find('[data-toggle="dropdown"]').bootstrapDropdown()
     var $button = $('button[data-toggle="dropdown"]')
+    $button[0].focus()
     // Key escape
-    $button.trigger('focus').trigger($.Event('keydown', {
-      which: 27
-    }))
+    var keydown = new Event('keydown')
+    keydown.which = 27
+    $button[0].dispatchEvent(keydown)
     assert.ok(!$dropdown.parent('.dropdown').hasClass('show'), 'dropdown menu is not shown after escape pressed')
     done()
   })