scripted-diff: Bump copyright headers

The previous diff touched most files in ./test/, so bump the headers to
avoid having to touch them again for a bump later.

-BEGIN VERIFY SCRIPT-
./contrib/devtools/copyright_header.py update ./test/
-END VERIFY SCRIPT-

test/lint/commit-script-check.sh

@@ -1,5 +1,5 @@
 #!/bin/sh
-# Copyright (c) 2017-2020 The Bitcoin Core developers
+# Copyright (c) 2017-2021 The Bitcoin Core developers
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.