test: avoid direct block header modification in feature_block.py

This is a preparatory commit for removing the header hash
caching in the CBlockHeader class. In order to not lose the
old block hash, necessary for updating the internal state of
the test (represented by `self.block_heights` and `self.blocks`),
we should only modify it within the `update_block` method.

Author: theStack

test/functional/feature_block.py

@@ -719,8 +719,7 @@ def run_test(self):
         # valid timestamp
         self.move_tip(53)
         b55 = self.next_block(55, spend=out[15])
-        b55.nTime = b35.nTime
-        self.update_block(55, [])
+        self.update_block(55, [], nTime=b35.nTime)
         self.send_blocks([b55], True)
         self.save_spendable_output()
 
@@ -1408,10 +1407,12 @@ def move_tip(self, number):
         self.tip = self.blocks[number]
 
     # adds transactions to the block and updates state
-    def update_block(self, block_number, new_transactions):
+    def update_block(self, block_number, new_transactions, *, nTime=None):
         block = self.blocks[block_number]
         self.add_transactions_to_block(block, new_transactions)
         old_sha256 = block.sha256
+        if nTime is not None:
+            block.nTime = nTime
         block.hashMerkleRoot = block.calc_merkle_root()
         block.solve()
         # Update the internal state just like in next_block