Support checking for asynchronous exceptions in Lwt tests

[craigfe]

The Lwt test runner currently catches asynchronous exceptions in tests and triggers a test failure. This is nice functionality, but doesn't allow for the use of check_raises to write tests that expect asynchronous exceptions to be thrown. It's clearly possible to write a handler manually via something like this:

  Lwt.catch
    (fun () -> f () >|= fun () -> `Ok)
    (function e -> Lwt.return @@ `Error e)
  >|= function
  | `Error Lib_exn -> Alcotest.(check pass) "Correct exception" () ()
  | `Error _       -> Alcotest.fail "Incorrect exception was thrown"
  | `Ok            -> Alcotest.fail "No exception was thrown"

but it seems to me that it would be nice for alcotest-lwt to provide users with an asynchronous variant of check_raises that hides this boilerplate.

It might be possible to solve #88 at the same time by changing the exception-handling API altogether.