fix: increase testnode network retries and add backoff (backport #6968) (#7114)

## Summary
- Increase `NewNetwork` max retries from 3 to 5 to give CI runners more
chances to find free ports
- Replace fixed 1s sleep with linear backoff (1s, 2s, 3s, 4s, 5s)
between retries
- Add a log line on each retry attempt for better CI debuggability

Addresses flaky CI failure in
`TestTxClientTestSuite/TestMultiConnBroadcast` where port binding errors
exhausted all retry attempts:
https://github.com/celestiaorg/celestia-app/actions/runs/24018301319/job/70041920378

## Backport notes
Mergify left literal `<<<<<<< HEAD` / `>>>>>>>` markers in
`test/util/testnode/network.go` because v8.x reorganized the retry loop
so that `cleanup()` runs inside the `isPortBindingError` branch (vs.
outside it on `main`). Conflict resolved by keeping the v8.x structure
and layering the new log line + linear backoff on top.

## Test plan
- [x] `go build ./test/util/testnode/...` compiles successfully
- [ ] CI passes on this PR

🤖 Generated with [Claude Code](https://claude.com/claude-code)


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<hr>This is an automatic backport of pull request #6968 done by
[Mergify](https://mergify.com).
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---------

Co-authored-by: Rootul P <rootulp@gmail.com>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: 3 people

test/util/testnode/network.go

@@ -20,7 +20,7 @@ import (
 // addresses. Configured genesis options will be applied after all accounts have
 // been initialized.
 func NewNetwork(t testing.TB, config *Config) (cctx Context, rpcAddr, grpcAddr string) {
-	return NewNetworkWithRetry(t, config, 3)
+	return NewNetworkWithRetry(t, config, 5)
 }
 
 // NewNetworkWithRetry creates a testnode network with port retry logic
@@ -34,7 +34,8 @@ func NewNetworkWithRetry(t testing.TB, config *Config, maxRetries int) (cctx Con
 				if cleanup != nil {
 					cleanup()
 				}
-				time.Sleep(time.Second)
+				t.Logf("port binding error on attempt %d/%d, retrying after %ds: %v", attempt+1, maxRetries, attempt+1, err)
+				time.Sleep(time.Duration(attempt+1) * time.Second)
 				config.TmConfig.RPC.ListenAddress = fmt.Sprintf("tcp://127.0.0.1:%d", MustGetFreePort())
 				config.TmConfig.P2P.ListenAddress = fmt.Sprintf("tcp://127.0.0.1:%d", MustGetFreePort())
 				config.TmConfig.RPC.GRPCListenAddress = fmt.Sprintf("tcp://127.0.0.1:%d", MustGetFreePort())