refactor(channels): dedupe transport and gateway test scaffolds

src/browser/test-port.ts

@@ -0,0 +1,18 @@
+import type { AddressInfo } from "node:net";
+import { createServer } from "node:http";
+
+export async function getFreePort(): Promise<number> {
+  while (true) {
+    const port = await new Promise<number>((resolve, reject) => {
+      const s = createServer();
+      s.once("error", reject);
+      s.listen(0, "127.0.0.1", () => {
+        const assigned = (s.address() as AddressInfo).port;
+        s.close((err) => (err ? reject(err) : resolve(assigned)));
+      });
+    });
+    if (port < 65535) {
+      return port;
+    }
+  }
+}