Allow skipping a test case from inside the test case

[apeschar]

This is useful when you want to perform logic to check whether or not some test case can run in the current environment.

let test_only_on_windows () =
  if Sys.os_type <> "Win32" then Alcotest.skip () else ();
  (* actual test code *)

[samoht]

Hi! Thanks for your contribution, that looks like a good idea!

Out of curiosity, in what context do you use Alcotest? Are you building interesting projects with it? I'm always interested to hear about new use-cases :-)