test: ensure that mining is possible without CL info (#5689)

## Issue being fixed or feature implemented
With DIP29 added to v20, miners include best CL Signature in CbTx.
The purpose of this test, is to ensure that mining is still possible
when CL information isn't available.
In such case, miners are expected to copy best CL Signature from CbTx of
previous block.

## What was done?
Two scenarios are implemented:

- Add dynamically a node, make sure `getbestchainlock()` fails, let it
mine a block.
- Disable `SPORK_19_CHAINLOCKS_ENABLED`, add dynamically a node, make
sure `getbestchainlock()` fails, let it mine a block.

In both tests, we make sure the block is accepted by everyone and that
the `bestCLSignature` in CbTx is copied from previous block.

## How Has This Been Tested?
`feature_llmq_chainlocks.py`

## Breaking Changes
no

## Checklist:
- [x] I have performed a self-review of my own code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [x] I have added or updated relevant unit/integration/functional/e2e
tests
- [ ] I have made corresponding changes to the documentation
- [x] I have assigned this pull request to a milestone _(for repository
code-owners and collaborators only)_

---------

Co-authored-by: UdjinM6 <UdjinM6@users.noreply.github.com>

test/functional/feature_llmq_chainlocks.py

@@ -13,7 +13,7 @@ Checks LLMQs based ChainLocks
 import time
 
 from test_framework.test_framework import DashTestFramework
-from test_framework.util import force_finish_mnsync, assert_equal
+from test_framework.util import force_finish_mnsync, assert_equal, assert_raises_rpc_error
 
 
 class LLMQChainLocksTest(DashTestFramework):
@@ -187,6 +187,24 @@ class LLMQChainLocksTest(DashTestFramework):
         self.reconnect_isolated_node(0, 1)
         self.wait_for_chainlocked_block(self.nodes[0], self.nodes[0].getbestblockhash(), timeout=30)
 
+        for i in range(2):
+            self.log.info(f"{'Disable' if i == 0 else 'Enable'} Chainlock")
+            self.nodes[0].sporkupdate("SPORK_19_CHAINLOCKS_ENABLED", 4070908800 if i == 0 else 0)
+            self.wait_for_sporks_same()
+
+            self.log.info("Add a new node and let it sync")
+            self.dynamically_add_masternode(evo=False)
+            added_idx = len(self.nodes) - 1
+            assert_raises_rpc_error(-32603, "Unable to find any chainlock", self.nodes[added_idx].getbestchainlock)
+
+            self.log.info("Test that new node can mine without Chainlock info")
+            tip_0 = self.nodes[0].getblock(self.nodes[0].getbestblockhash(), 2)
+            self.nodes[added_idx].generate(1)
+            self.sync_blocks(self.nodes)
+            tip_1 = self.nodes[0].getblock(self.nodes[0].getbestblockhash(), 2)
+            assert_equal(tip_1['cbTx']['bestCLSignature'], tip_0['cbTx']['bestCLSignature'])
+            assert_equal(tip_1['cbTx']['bestCLHeightDiff'], tip_0['cbTx']['bestCLHeightDiff'] + 1)
+
     def create_chained_txs(self, node, amount):
         txid = node.sendtoaddress(node.getnewaddress(), amount)
         tx = node.getrawtransaction(txid, 1)