chore: fix test suite on macOS (#12780)

MacOS uses a very old version of bash where `"${FOO[@]}"` fails if `set
-u` is enabled and `FOO` is undefined. Newer versions of bash expand
this to zero arguments instead.

Also, `lint.py` used the shebang `#!/usr/bin/env python` instead of
`python3`, which fails on some systems.

In CI, all macos tests run on nscloud runners. Presumably, they have
installed newer versions of various software, hence this didn't break in
CI.

script/gen_constants_cpp.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 #
 # Copyright (c) 2015 Microsoft Corporation. All rights reserved.