Separate stateless verification mode from other modes and use a dedicated "testing mode" for it

[gustavo-grieco]

Stateless verification does not need corpus replay nor multiple workers, so it can be separated from the other features that require specific worker interaction. Ideally: echidna . --contract Test --test-mode verification should be enough to verify stateless properties.

Related with #1490

[gustavo-grieco]

This is already implemented in #1502, but needs to be reviewed carefully as it is a big change