test: add blocks_key_path

Adds a convenience function to TestNode
to provide the path to the blocks xor key.
Updates util and feature_blocksxor to use it.

test/functional/test_framework/test_node.py

@@ -465,6 +465,10 @@ class TestNode():
     def blocks_path(self) -> Path:
         return self.chain_path / "blocks"
 
+    @property
+    def blocks_key_path(self) -> Path:
+        return self.blocks_path / "xor.dat"
+
     @property
     def wallets_path(self) -> Path:
         return self.chain_path / "wallets"