MDL-81608 core: Add alt text to the YUI spinner image

The accessibility Behat test added to the quiz editing page uncovered
that the loading spinner icon does not have an appropriate alt text.

public/lib/javascript-static.js

@@ -826,6 +826,7 @@ M.util.add_spinner = function(Y, node) {
 
     var spinner = Y.Node.create('<img />')
         .setAttribute('src', M.util.image_url(WAITICON.pix, WAITICON.component))
+        .setAttribute('alt', M.util.get_string('loading', 'core'))
         .addClass('spinner icon')
         .hide();