test: increase specificity in dgram test

Expand error message checking to include the entire error string in
test-dgram-membership.

PR-URL: nodejs#11187
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/parallel/test-dgram-membership.js

@@ -14,7 +14,7 @@ const setup = () => {
   const socket = setup();
   socket.close(common.mustCall(() => {
     assert.throws(() => { socket.addMembership(multicastAddress); },
-                  /Not running/);
+                  /^Error: Not running$/);
   }));
 }
 
@@ -23,23 +23,23 @@ const setup = () => {
   const socket = setup();
   socket.close(common.mustCall(() => {
     assert.throws(() => { socket.dropMembership(multicastAddress); },
-                  /Not running/);
+                  /^Error: Not running$/);
   }));
 }
 
 // addMembership() with no argument should throw
 {
   const socket = setup();
   assert.throws(() => { socket.addMembership(); },
-                /multicast address must be specified/);
+                /^Error: multicast address must be specified$/);
   socket.close();
 }
 
 // dropMembership() with no argument should throw
 {
   const socket = setup();
   assert.throws(() => { socket.dropMembership(); },
-                /multicast address must be specified/);
+                /^Error: multicast address must be specified$/);
   socket.close();
 }
 
@@ -69,7 +69,7 @@ const setup = () => {
   const socket = setup();
   assert.throws(
     () => { socket.dropMembership(multicastAddress); },
-    /EADDRNOTAVAIL/
+    /^Error: dropMembership EADDRNOTAVAIL$/
   );
   socket.close();
 }