Avoid overwritting echidna config in slither-prop

The echidna config file is saved with the same name, in the same directory, each time `slither-prop` is called:

https://github.com/crytic/slither/blob/4319bb360542cab6a89d9bacd039d4c70b8b76e5/slither/tools/properties/platforms/echidna.py#L19

Ideally, we should create a directory and rename the file according the contract, the properties and the scenario it tests.