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_reindex.py

@@ -10,7 +10,6 @@
 - Verify that out-of-order blocks are correctly processed, see LoadExternalBlockFile()
 """
 
-import os
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.p2p import MAGIC_BYTES
 from test_framework.util import assert_equal
@@ -39,7 +38,7 @@ class ReindexTest(BitcoinTestFramework):
         # In this test environment, blocks will always be in order (since
         # we're generating them rather than getting them from peers), so to
         # test out-of-order handling, swap blocks 1 and 2 on disk.
-        blk0 = os.path.join(self.nodes[0].datadir, self.nodes[0].chain, 'blocks', 'blk00000.dat')
+        blk0 = self.nodes[0].chain_path / "blocks" / "blk00000.dat"
         with open(blk0, 'r+b') as bf:
             # Read at least the first few blocks (including genesis)
             b = bf.read(2000)