test: fix flaky test-cluster-shared-leak

Swallow EPIPE as there is it is expected to come up from time to time.
This does not invalidate the test.

Fixes: nodejs#3956
PR-URL: nodejs#4173

Author: Trott

test/parallel/parallel.status

@@ -7,7 +7,6 @@ prefix parallel
 [true] # This section applies to all platforms
 
 [$system==win32]
-test-cluster-shared-leak             : PASS,FLAKY
 test-debug-no-context                : PASS,FLAKY
 test-tls-ticket-cluster              : PASS,FLAKY
 

test/parallel/test-cluster-shared-leak.js

@@ -18,6 +18,7 @@ if (cluster.isMaster) {
     conn = net.connect(common.PORT, common.mustCall(function() {
       worker1.send('die');
       worker2.send('die');
+      worker2.on('error', function(e) {});
     }));
     conn.on('error', function(e) {
       // ECONNRESET is OK