Add flag: returnBool for property testing

[montyly]

returnBool set to false will make medusa accept properties that don't return a boolean.

This is meant to mimic foundry's behavior, and ease the compatibility between the two tools. See https://getfoundry.sh/forge/advanced-testing/invariant-testing/#conditional-invariants

[elopez]

Thanks for the PR! I'm overall confused about the feature though, wouldn't a property that does not return a boolean not be a property? It sounds like it would be something for the assertion testing mode instead.

[elopez]

but maybe it needs to be unified the other way to testReturnBool? it seems JSON options match their Go counterparts.

[montyly]

@elopez the way I have seen people using it in foundry setup is basically that the invariant_ functions should not revert.

It would be the same as having require(return_variable)