fix(cli-exec): poll healthcheck in 'can start on an alternate port' test

`start(['--port=4567'])` returns a Promise that resolves on process exit,
not on listen — the test fired a single healthcheck immediately and got
ECONNREFUSED on slow Windows runners before the alternate-port server
had bound. Replace the single shot with a 5-second poll loop, mirroring
the pattern already used in the 'stops the process when terminated' test.

Eliminates the intermittent ECONNREFUSED 127.0.0.1:4567 observed on
PR #2172 run 24120410492 (Test @percy/cli-exec, Windows).

Author: Shivanshu-07

packages/cli-exec/test/start.test.js

@@ -71,7 +71,19 @@ describe('percy exec:start', () => {
 
   it('can start on an alternate port', async () => {
     start(['--quiet', '--port=4567']);
-    let response = await request('http://localhost:4567/percy/healthcheck');
+    // `start(...)` resolves only when the percy process exits, not when the
+    // server has bound to the port. Polling healthcheck instead of a single
+    // shot avoids ECONNREFUSED on slow runners (notably windows-latest)
+    // before the listener is up.
+    let response = await (async function poll(deadline = Date.now() + 5000) {
+      try {
+        return await request('http://localhost:4567/percy/healthcheck');
+      } catch (err) {
+        if (Date.now() >= deadline) throw err;
+        await new Promise(r => setTimeout(r, 100));
+        return poll(deadline);
+      }
+    })();
     expect(response).toHaveProperty('success', true);
   });