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/util.py

@@ -516,7 +516,7 @@ def check_node_connections(*, node, num_in, num_out):
 
 
 def read_xor_key(*, node):
-    with open(node.blocks_path / "xor.dat", "rb") as xor_f:
+    with open(node.blocks_key_path, "rb") as xor_f:
         NUM_XOR_BYTES = 8 # From InitBlocksdirXorKey::xor_key.size()
         return xor_f.read(NUM_XOR_BYTES)