Fail instead of looping when data cannot be generated (bad_test hit too often)

[smelc]

If a test is too restrictive:

let is_bad_data : ... -> bool

f data =
  if is_bad_data data
  then Crowbar.bad_test ()
  else ... (* do test *)

let () =
  Crowbar.add_test ~name:"test f" [generator] f

I mean if Crowbar.bad_test is hit some number of times, it would be better to fail at some point, to give feedback to the caller. At the time of writing, Crowbar doesn't terminate. For example Haskell's quickcheck does this, along the lines of giving up generating data after X trials and only Y successes.

As this is a breaking change (it could make tests that pass now (slowly though) fail), care should be taken regarding adoption.

Thanks for your work on crowbar ❤️