integration: scale remaining hardcoded timeouts and replace pingAllHelper

Apply CI-aware scaling to all remaining hardcoded timeouts:

- requireAllClientsOfflineStaged: scale the three internal stage
  timeouts (15s/20s/60s) with ScaledTimeout.
- validateReloginComplete: scale requireAllClientsOnline (120s)
  and requireAllClientsNetInfoAndDERP (3min) calls.
- WaitForTailscaleSyncPerUser callers in acl_test.go (3 sites, 60s).
- WaitForRunning callers in tags_test.go (10 sites): switch to
  PeerSyncTimeout() to match convention.
- WaitForRunning/WaitForPeers direct callers in route_test.go.
- requireAllClientsOnline callers in general_test.go and
  auth_key_test.go.

Replace pingAllHelper with assertPingAll/assertPingAllWithCollect:

- Wraps pings in EventuallyWithT so transient docker exec timeouts
  are retried instead of immediately failing the test.
- Timeout scales with the ping matrix size (2s per ping budget for
  2 full sweeps) so large tests get proportionally more time.
- Uses CollectT correctly, fixing the broken EventuallyWithT usage
  in TestEphemeral where the old t.Errorf bypassed CollectT.
- Follows the established assert*/assertWithCollect naming.

Updates #3125

integration/route_test.go

@@ -2430,7 +2430,7 @@ func TestAutoApproveMultiNetwork(t *testing.T) {
 
 					// Wait for the node to be fully running before getting its ID
 					// This is especially important for webauth flow where login is asynchronous
-					err = routerUsernet1.WaitForRunning(30 * time.Second)
+					err = routerUsernet1.WaitForRunning(integrationutil.ScaledTimeout(30 * time.Second))
 					require.NoError(t, err)
 
 					// Wait for bidirectional peer synchronization.
@@ -2439,12 +2439,12 @@ func TestAutoApproveMultiNetwork(t *testing.T) {
 					// tunnels may not be established despite peers appearing in netmaps.
 
 					// Router waits for all existing clients
-					err = routerUsernet1.WaitForPeers(len(allClients), 60*time.Second, 1*time.Second)
+					err = routerUsernet1.WaitForPeers(len(allClients), integrationutil.PeerSyncTimeout(), integrationutil.PeerSyncRetryInterval())
 					require.NoError(t, err, "router failed to see all peers")
 
 					// All clients wait for the router (they should see 6 peers including the router)
 					for _, existingClient := range allClients {
-						err = existingClient.WaitForPeers(len(allClients), 60*time.Second, 1*time.Second)
+						err = existingClient.WaitForPeers(len(allClients), integrationutil.PeerSyncTimeout(), integrationutil.PeerSyncRetryInterval())
 						require.NoErrorf(t, err, "client %s failed to see all peers including router", existingClient.Hostname())
 					}