This adds a test for create_ticket_run, which is complex because
create_ticket_run calls into other parts of the gvmd code (copy_ticket,
ticket_uuid, log_event, etc).

The test creates 'mock' versions of those functions.

A gcc attribute is used as an easy way to override the mocked functions.
The alternative would be to link against less of the gvmd code and to
fill the gaps with stub functions.  But our code is not very cleanly
separated to this is a lot of work.  Should get easier as we clean
things up, if the attribute is not suitable.

This test requires a system function to be wrapped, and specifies the
return of the system function in the test (which is more usefull than
specifying it in the wrapper).