[client] Fix flaky Test_ConnectPeers busy-loop handshake wait (#6871)

## Describe your changes

  `Test_ConnectPeers` in `client/iface` is flaky in CI, timing out with
  `waiting for peer handshake timeout after 30s`.

  The wait loop polled `getPeer()` in a tight busy-loop with no sleep
(a `select` with a `default` branch), pegging a CPU core. The peers run
  userspace WireGuard (stdnet transport), so the spin starved the
wireguard-go goroutines that actually process the handshake, making the
  30s wait flaky under CI load.

Poll on a 500ms ticker instead so the CPU is yielded between checks, and
  check the handshake state before waiting. Same logic, no busy-spin.

  ## Issue ticket number and link

  N/A — CI flakiness fix.

  ### Checklist
  - [x] Is it a bug fix
  - [ ] Is a typo/documentation fix
  - [ ] Is a feature enhancement
  - [ ] It is a refactor
  - [ ] Created tests that fail without the change (if possible)
- [x] This change does **not** modify the public API, gRPC protocols,
functionality behavior, CLI / service flags, or introduce a new feature

  ## Documentation

  - [x] Documentation is **not needed** for this change (test-only fix)

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<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Tests**
* Improved peer connection test waiting behavior to avoid busy spinning.
* Added a timeout and periodic checks, with clearer failure handling
when a handshake does not complete.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

client/iface/iface_test.go

@@ -569,17 +569,17 @@ func Test_ConnectPeers(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	// todo: investigate why in some tests execution we need 30s
+	// The peers use userspace WireGuard (stdnet transport). A tight busy-loop
+	// here starves the wireguard-go goroutines that process the handshake, so
+	// poll on a ticker instead and yield the CPU between checks. WireGuard also
+	// only retries a lost handshake initiation every REKEY_TIMEOUT (5s), which
+	// is why the overall wait can occasionally stretch to tens of seconds.
 	timeout := 30 * time.Second
 	timeoutChannel := time.After(timeout)
+	ticker := time.NewTicker(500 * time.Millisecond)
+	defer ticker.Stop()
 
 	for {
-		select {
-		case <-timeoutChannel:
-			t.Fatalf("waiting for peer handshake timeout after %s", timeout.String())
-		default:
-		}
-
 		peer, gpErr := getPeer(peer1ifaceName, peer2Key.PublicKey().String())
 		if gpErr != nil {
 			t.Fatal(gpErr)
@@ -588,6 +588,12 @@ func Test_ConnectPeers(t *testing.T) {
 			t.Log("peers successfully handshake")
 			break
 		}
+
+		select {
+		case <-timeoutChannel:
+			t.Fatalf("waiting for peer handshake timeout after %s", timeout.String())
+		case <-ticker.C:
+		}
 	}
 
 }