test: Use wallet_dir lambda in wallet_multiwallet test where possible

Seems odd to hardcode all parent directory names in the path for no good
reason.

Also, add wallet_path property to TestNode.

Also, rework wallet_backup.py test for scripted-diff in the next commit.

test/functional/test_framework/test_node.py

@@ -413,6 +413,10 @@ class TestNode():
     def debug_log_path(self) -> Path:
         return self.chain_path / 'debug.log'
 
+    @property
+    def wallets_path(self) -> Path:
+        return self.chain_path / "wallets"
+
     def debug_log_bytes(self) -> int:
         with open(self.debug_log_path, encoding='utf-8') as dl:
             dl.seek(0, 2)