Add an optional argument for protecting the run of the tests

[n-osborne]

This PR propose to add a -p optional argument to the Ortac/QCheck-STM
command-line.

The new optional argument takes a string which should be the name of the function to call.
The function should be implemented in the module passed to the -i option.
The funciton has type (unit -> unit) -> unit, which means that running the tests is suspended so the basic usage is:

let run_tests tests = (* do something *) (tests ())

[n-osborne]

Note to myself: I need to add the equivalent optional argument to Ortac/Dune plugin.

[n-osborne]

Converting to draft as it is not up to date with the module-based configuration from #214