testnode doesn't support custom chain IDs

[rootulp]

Context

Integration tests are failing in celestiaorg/celestia-node#3453

Problem

I discovered that there is a bug in testnode that prevents it from starting a network with a custom chain ID. Confirmed with this unit test:

package app_test

import (
	"testing"

	"github.com/celestiaorg/celestia-app/v2/test/util/testnode"
)

func Test_testnode(t *testing.T) {
	t.Run("testnode can start a network with default config", func(t *testing.T) {
		testnode.NewNetwork(t, testnode.DefaultConfig())
	})
	t.Run("testnode can start a network with a custom chain ID", func(t *testing.T) {
		config := testnode.DefaultConfig()
		config.Genesis.ChainID = "foo"
		testnode.NewNetwork(t, config)
	})
}

where the second test case fails. I think this is a regression because in v1.x the integration tests in celestia-node passed and they used testnode from celestia-app.

Proposal

1. Fix testnode so that it can support custom chain IDs. celestia-node spins up swamp tests with the chain ID "private".
2. Backport the fix to v2.x
3. Cut another release candidate
4. Bump to that release candidate in celestia-node PR