Add testing for removing a gadget which should remove both its DOM and

GadgetIndex representation.
Adding a new tab should rmove existing gadget and its representation.

test/loading/A.html

+test!!!
\ No newline at end of file

test/loading/multiple-gadgets.html

+<div id="A"
+     data-gadget="loading/A.html"></div>
+
+<div id="B"
+     data-gadget="loading/A.html"></div>
\ No newline at end of file