Changes

tsk71 Adds tests to delete story method
Adds missing tests to delete story method. These tests probe that an
user can't remove another user stories, an unlogged user can't remove
any story and that if an user tries to delete a story that couldn't be
found, an error was thrown. These tests are added on both user and story
resources. Also a new auxiliary method was created to check in the tests
that an internal server error is received.