test: Use TestNode *_path properties where possible

Seems odd to place the burden on test writers to hardcode the chain or
datadir path for the nodes under test.

test/functional/feature_posix_fs_permissions.py

@@ -31,11 +31,11 @@ class PosixFsPermissionsTest(BitcoinTestFramework):
 
     def run_test(self):
         self.stop_node(0)
-        datadir = os.path.join(self.nodes[0].datadir, self.chain)
+        datadir = self.nodes[0].chain_path
         self.check_directory_permissions(datadir)
-        walletsdir = os.path.join(datadir, "wallets")
+        walletsdir = self.nodes[0].wallets_path
         self.check_directory_permissions(walletsdir)
-        debuglog = os.path.join(datadir, "debug.log")
+        debuglog = self.nodes[0].debug_log_path
         self.check_file_permissions(debuglog)