Added a check to prevent slots from being used when verifying multiple calls

[Raibaz]

This checks if a verify block contains the same function more than once and slots are being used to capture its arguments.

If this is the case, the captured property of the slot will contain only the argument of the last call, with the tests not behaving as expected.

An exception is now thrown in this case, to suggest using a mutableList to capture arguments.

This should fix #352 by clarifying to users the right way to capture multiple invocations of the same function.