jquery.makeCollapsible: use 'mw-collapsible' event namespace

For consistency with the module name.

Change-Id: Ied13e5abdec107771e80418feb4bb3b2f0d54567
jquery.makeCollapsible: basic test suite

For now it just verifies that the very basic synchronous functionality
of the module works; not much more is possible without triggering some
events to hook to, and currently jquery.makeCollapsible doesn't
trigger any.

Change-Id: Icdd8f392e4921007a6bdfb3ae934949e4c875232
Merge changes Icdd8f392,Ied13e5ab

* changes:
  jquery.makeCollapsible: basic test suite
  jquery.makeCollapsible: use 'mw-collapsible' event namespace