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.

Author: katesalazar

test/functional/test_runner.py

@@ -344,7 +344,7 @@ 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()