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

test/functional/feature_logging.py

@@ -16,7 +16,7 @@ class LoggingTest(BitcoinTestFramework):
         self.setup_clean_chain = True
 
     def relative_log_path(self, name):
-        return os.path.join(self.nodes[0].datadir, "regtest", name)
+        return os.path.join(self.nodes[0].datadir, self.chain, name)
 
     def run_test(self):
         # test default log file name