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_loadblock.py

@@ -37,7 +37,7 @@ class LoadblockTest(BitcoinTestFramework):
         cfg_file = os.path.join(data_dir, "linearize.cfg")
         bootstrap_file = os.path.join(self.options.tmpdir, "bootstrap.dat")
         genesis_block = self.nodes[0].getblockhash(0)
-        blocks_dir = os.path.join(data_dir, self.chain, "blocks")
+        blocks_dir = self.nodes[0].chain_path / "blocks"
         hash_list = tempfile.NamedTemporaryFile(dir=data_dir,
                                                 mode='w',
                                                 delete=False,