Calling cxy_cache_sup:start_cache/3 succeeds with bad param

[jaynel]

The third argument should be a function, and the code indeed does is_function(Gen_Fun, 3), but the call does not crash.

Create a test case passing the wrong type argument for the third arg (the example was 1000) and ensure that the cxy_cache_fsm:start_link/3 fails when the arglist is wrong.