ping6: Use errx to avoid appending a specious error message

Reviewed by:	asomers, markj
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D38418

sbin/ping/ping6.c

@@ -580,7 +580,7 @@ ping6(int argc, char *argv[])
 		case 'W':
 			t = strtod(optarg, &e);
 			if (*e || e == optarg || t > (double)INT_MAX)
-				err(EX_USAGE, "invalid timing interval: `%s'",
+				errx(EX_USAGE, "invalid timing interval: `%s'",
 				    optarg);
 			options |= F_WAITTIME;
 			waittime = (int)t;

sbin/ping/tests/test_ping.py

@@ -736,7 +736,6 @@ def test_ping(self, expected):
                 "stdout": "",
                 "stderr": "ping: invalid timing interval: `x'\n",
             },
-            marks=pytest.mark.skip("XXX currently failing"),
             id="_Wx_localhost",
         ),
     ]