test: deflake test-net-listen-ipv6only

The test verified ipv6Only by connecting to the IPv4 side of an
ephemeral port and expecting ECONNREFUSED, but it never reserved that
IPv4 port. Under parallel execution another test could occupy it,
making the connection succeed instead of being refused.

Run the test sequentially with a fixed common.PORT so it no longer
competes with other tests for the same port, matching the fix already
applied to the sibling cluster variants.

Fixes: #64172
Signed-off-by: sangwook <rewq5991@gmail.com>

Author: Han5991

test/parallel/test-net-listen-ipv6only.js …t/sequential/test-net-listen-ipv6only.jstest/parallel/test-net-listen-ipv6only.js renamed to test/sequential/test-net-listen-ipv6only.js test/parallel/test-net-listen-ipv6only.js renamed to test/sequential/test-net-listen-ipv6only.js

@@ -11,9 +11,13 @@ const net = require('net');
 
 const host = '::';
 const server = net.createServer();
+// Use a fixed port and run this test sequentially. The assertion below relies
+// on nothing else listening on the IPv4 side of the chosen port; with an
+// ephemeral port under parallel execution another test can occupy that IPv4
+// port, making the connection succeed instead of being refused.
 server.listen({
   host,
-  port: 0,
+  port: common.PORT,
   ipv6Only: true,
 }, common.mustCall(() => {
   const { port } = server.address();