Tests: Use self.chain instead of 'regtest' in almost all current tests

test/functional/rpc_scantxoutset.py

@@ -54,7 +54,7 @@ class ScantxoutsetTest(BitcoinTestFramework):
 
         self.log.info("Stop node, remove wallet, mine again some blocks...")
         self.stop_node(0)
-        shutil.rmtree(os.path.join(self.nodes[0].datadir, "regtest", 'wallets'))
+        shutil.rmtree(os.path.join(self.nodes[0].datadir, self.chain, 'wallets'))
         self.start_node(0)
         self.nodes[0].generate(110)