LoadingMessageSourceProviderTest: add a failing test...

Spotted here:

http://codereview.stackexchange.com/questions/27452/review-custom-simple-cache-with-load-timeout-and-expiry-thread-safety-tests

The scenario is as follows for one task t:

Expiry            getMessageSource
---------         ---------
                  lock sources
                  grab task t
                  unlock sources
                  t.get()
                  <t.get() working>
lock sources
grab task t
cancel task t
                  BOOM: CancellationException