test: don't skip when common.mustCall() is pending

The test parallel/test-dgram-multicast-set-interface.js was
calling common.skip() on hosts that do not support IPv6. However,
by this point, there were several outstanding common.mustCall()
invocations. The process.exit() in common.skip() triggered
those common.mustCall()s as errors.

Fixes: https://github.com/nodejs/node/issues/15419
PR-URL: https://github.com/nodejs/node/pull/15421
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

test/parallel/test-dgram-multicast-set-interface.js

@@ -72,10 +72,11 @@ const dgram = require('dgram');
   }));
 }
 
-if (!common.hasIPv6) {
+// If IPv6 is not supported, skip the rest of the test. However, don't call
-  common.skip('Skipping udp6 tests, no IPv6 support.');
+// common.skip(), which calls process.exit() while there is outstanding
+// common.mustCall() activity.
+if (!common.hasIPv6)
   return;
-}
 
 {
   const socket = dgram.createSocket('udp6');