spot.ltsmin.require('divine') should check the exit status

Currently it does

            if shutil.which("divine") == None:
                print("divine not available", file=sys.stderr)
                sys.exit(77)
            out = subprocess.check_output(['divine', 'compile', '--help'],
                                          stderr=subprocess.STDOUT)
            if b'LTSmin' not in out:
                print("divine available but no support for LTSmin",
                       file=sys.stderr)
                sys.exit(77)

However if divine compile --help terminates with a non-zero exit code (as in the current version of divine), this triggers an exception and that causes our tests to fail instead of being skipped.

Reported by @xlauko.