Merge bitcoin/bitcoin#32742: test: fix catchup loop in outbound eviction functional test

dd8447f test: fix catchup loop in outbound eviction functional test (Sebastian Falbesoner)

Pull request description:

  In the course of working on an equivalent of #32421 for the `CBlockHeader` class, I noticed that the [catchup loop in the outbound eviction functional test](https://github.com/bitcoin/bitcoin/blob/19765dca197a6334e5ad3985a30c7836374d5c26/test/functional/p2p_outbound_eviction.py#L86-L103) currently has a small flaw: the contained waiting for a `getheaders` message

  https://github.com/bitcoin/bitcoin/blob/19765dca197a6334e5ad3985a30c7836374d5c26/test/functional/p2p_outbound_eviction.py#L98-L99

  only waits for _any_ such message instead of one with the intended block hash after the first iteration. The reason is that the `prev_prev_hash` variable is set incorrectly, since the `tip_header` instance is not updated and its field `.hash` is None [1]. Fix that by updating `tip_header` after generating a new block and also use the correct field on it -- we want the tip header's previous hash (`.hashPrevBlock`), which will be the previous-previous hash in the next iteration as intended.

  Can be demonstrated by adding a debug output for `prev_prev_hash`, e.g.

  ```diff
  diff --git a/test/functional/p2p_outbound_eviction.py b/test/functional/p2p_outbound_eviction.py
  index 30ac85e..9886a49512 100755
  --- a/test/functional/p2p_outbound_eviction.py
  +++ b/test/functional/p2p_outbound_eviction.py
  @@ -85,6 +85,7 @@ class P2POutEvict(BitcoinTestFramework):

           self.log.info("Keep catching up with the old tip and check that we are not evicted")
           for i in range(10):
  +            print(f"i={i}, prev_prev_hash={prev_prev_hash}")
               # Generate an additional block so the peers is 2 blocks behind
               prev_header = from_hex(CBlockHeader(), node.getblockheader(best_block_hash, False))
               best_block_hash = self.generateblock(node, output="raw(42)", transactions=[])["hash"]
  ```

  master branch
  ```
  ...
  i=0, prev_prev_hash=21722572577213525620063947414919931742473663114977483853465070858884938201585
  i=1, prev_prev_hash=None
  i=2, prev_prev_hash=None
  i=3, prev_prev_hash=None
  i=4, prev_prev_hash=None
  i=5, prev_prev_hash=None
  i=6, prev_prev_hash=None
  i=7, prev_prev_hash=None
  i=8, prev_prev_hash=None
  i=9, prev_prev_hash=None
  ...
  ```

  PR branch
  ```
  ...
  i=0, prev_prev_hash=21722572577213525620063947414919931742473663114977483853465070858884938201585
  i=1, prev_prev_hash=23204083306104595181276643925327085197417756603258684897360269464191973063397
  i=2, prev_prev_hash=18117007775254206852722585270408843074799046031613422902091537272077477361634
  i=3, prev_prev_hash=30556804635951812756130312631227721973553160707632138130845362630877961299882
  i=4, prev_prev_hash=16476515948153779819467376247405243058769281687868039119037064816106574626111
  i=5, prev_prev_hash=14965506521435221774966695805624206855826023174786191695076697927307467053159
  i=6, prev_prev_hash=14510815979277079515923749862202324542606166669768865640616202929053689167149
  i=7, prev_prev_hash=15360268707191667685151951417759114642582372006627142890517655217275478262166
  i=8, prev_prev_hash=55984929479619644661389829786223559362979512070332438490054115824374865094074
  i=9, prev_prev_hash=6591573629906616262191232272909118561529534571119028248829355592878183757083
  ...
  ```

  [1] that's in my opinion another example how caching hashes is confusing and easy to be misused; it's better to remove it and just compute the hash on-the-fly, so returning None is not even possible anymore

ACKs for top commit:
  maflcko:
    lgtm ACK dd8447f
  mzumsande:
    Code Review ACK dd8447f
  pablomartin4btc:
    cr-ACK dd8447f

Tree-SHA512: bd8e786b52e3e96661453006140d6b8fad5a35f1c8d38243c61df52b19c97cd3800404745a2f9603bcdf0006e9780b4f15f8f7e4fa34ff07d52dba04d87b68d0

Author: glozow

test/functional/p2p_outbound_eviction.py

@@ -88,6 +88,7 @@ def test_outbound_eviction_unprotected(self):
             # Generate an additional block so the peers is 2 blocks behind
             prev_header = from_hex(CBlockHeader(), node.getblockheader(best_block_hash, False))
             best_block_hash = self.generateblock(node, output="raw(42)", transactions=[])["hash"]
+            tip_header = from_hex(CBlockHeader(), node.getblockheader(best_block_hash, False))
             peer.sync_with_ping()
 
             # Advance time but not enough to evict the peer
@@ -100,7 +101,7 @@ def test_outbound_eviction_unprotected(self):
 
             # Send a header with the previous tip (so we go back to 1 block behind)
             peer.send_and_ping(msg_headers([prev_header]))
-            prev_prev_hash = tip_header.hash
+            prev_prev_hash = tip_header.hashPrevBlock
 
         self.log.info("Create an outbound connection and take some time to catch up, but do it in time")
         # Check that if the peer manages to catch up within time, the timeouts are removed (and the peer is not disconnected)