test: replace 'regtest' leftovers by self.chain

Commit 1abcecc40c replaced 'regtest' by self.chain
'regtest' "in almost all current tests", this commit takes care of the remaining
ones.

test/functional/rpc_dumptxoutset.py

@@ -25,7 +25,7 @@ class DumptxoutsetTest(BitcoinTestFramework):
 
         FILENAME = 'txoutset.dat'
         out = node.dumptxoutset(FILENAME)
-        expected_path = Path(node.datadir) / 'regtest' / FILENAME
+        expected_path = Path(node.datadir) / self.chain / FILENAME
 
         assert expected_path.is_file()