test: Update --tmpdir doc string to say directory must not exist

The error message given if passing an existing dir to --tmpdir is
confusing so this makes it clear that the directory must not already
exist

test/functional/test_framework/test_framework.py

@@ -163,7 +163,7 @@ def parse_args(self):
                             help="Don't stop bitcoinds after the test execution")
         parser.add_argument("--cachedir", dest="cachedir", default=os.path.abspath(os.path.dirname(os.path.realpath(__file__)) + "/../../cache"),
                             help="Directory for caching pregenerated datadirs (default: %(default)s)")
-        parser.add_argument("--tmpdir", dest="tmpdir", help="Root directory for datadirs")
+        parser.add_argument("--tmpdir", dest="tmpdir", help="Root directory for datadirs (must not exist)")
         parser.add_argument("-l", "--loglevel", dest="loglevel", default="INFO",
                             help="log events at this level and higher to the console. Can be set to DEBUG, INFO, WARNING, ERROR or CRITICAL. Passing --loglevel DEBUG will output all logs to console. Note that logs at all levels are always written to the test_framework.log file in the temporary test directory.")
         parser.add_argument("--tracerpc", dest="trace_rpc", default=False, action="store_true",