test: don't assume IPv6 in test-regress-GH-5727

test/parallel/test-regress-GH-5727 assumed that one of the
servers would be listening on IPv6. This breaks when the machine
running the test doesn't have IPv6. This commit builds the
connection key that is compared dynamically.

Refs: #5732
PR-URL: #6319
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>

test/parallel/test-regress-GH-5727.js

@@ -7,7 +7,10 @@ const invalidPort = -1 >>> 0;
 const errorMessage = /"port" argument must be \>= 0 and \< 65536/;
 
 net.Server().listen(common.PORT, function() {
-  assert.equal(this._connectionKey, '6::::' + common.PORT);
+  const address = this.address();
+  const key = `${address.family.slice(-1)}:${address.address}:${common.PORT}`;
+
+  assert.equal(this._connectionKey, key);
   this.close();
 });