message: Add an argparse abbreviated mode to --failfast
  Short options should only be a single character. If not, they can't be
  concatenated in a single "-word" (from review by luke-jr).
  F is chosen instead of f, because f could be reserved to the nested
  wallet_hd.py (test_framework/test_framework.py) arguments parser.
diff: @@ -345,7 +345,7 @@ def main():
  parser.add_argument('--keepcache', '-k', action='store_true', help='the default behavior is to flush the cache directory on startup. --keepcache retains the cache from the previous testrun.')
  parser.add_argument('--quiet', '-q', action='store_true', help='only print dots, results summary and failure logs')
  parser.add_argument('--tmpdirprefix', '-t', default=tempfile.gettempdir(), help="Root directory for datadirs")
  - parser.add_argument('--failfast', action='store_true', help='stop execution after the first test failure')
  + parser.add_argument('--failfast', '-F', action='store_true', help='stop execution after the first test failure')
  parser.add_argument('--filter', help='filter scripts to run by regular expression')
  args, unknown_args = parser.parse_known_args()
  