test: reduce flakiness on test-net-write-fully-async-buffer

PR-URL: #56971
Refs: https://github.com/nodejs/node/actions/runs/13220154720/job/36904132463?pr=56970
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Marco Ippolito <marcoippolito54@gmail.com>

test/parallel/test-net-write-fully-async-buffer.js

@@ -23,7 +23,7 @@ const server = net.createServer(common.mustCall(function(conn) {
       }
 
       while (conn.write(Buffer.from(data)));
-      globalThis.gc({ type: 'minor' });
+      globalThis.gc({ type: 'major' });
       // The buffer allocated above should still be alive.
     }