Improve blocksdir functional test.

A new node should not create an unused `blocks` directory in the root of
the data directory when `-testnet` or `-regtest` is specified.

test/functional/feature_blocksdir.py

@@ -18,6 +18,8 @@ class BlocksdirTest(BitcoinTestFramework):
 
     def run_test(self):
         self.stop_node(0)
+        assert os.path.isdir(os.path.join(self.nodes[0].datadir, "regtest", "blocks"))
+        assert not os.path.isdir(os.path.join(self.nodes[0].datadir, "blocks"))
         shutil.rmtree(self.nodes[0].datadir)
         initialize_datadir(self.options.tmpdir, 0)
         self.log.info("Starting with nonexistent blocksdir ...")