Testmode: use ipv4 address to reduce dependency on local system configuration (#66968)

We've run into numerous issues with local system configurations that
prefer ipv6 and failing to connect to the test proxy.

---------

Co-authored-by: Zack Tanner <1939140+ztanner@users.noreply.github.com>

packages/next/src/experimental/testmode/proxy/server.ts

@@ -58,7 +58,7 @@ export async function createProxyServer({
   })
 
   await new Promise((resolve) => {
-    server.listen(0, 'localhost', () => {
+    server.listen(0, '::', () => {
       resolve(undefined)
     })
   })