devtools: Improve *-check.py tool detection

This is important to make sure that we're not testing tools different
from the one we're building with.

Introduce determine_wellknown_cmd, which encapsulates how we
should handle well-known tools specification (IFS splitting, env
override, etc.).

contrib/devtools/utils.py

@@ -0,0 +1,22 @@
+#!/usr/bin/env python3
+# Copyright (c) 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.
+'''
+Common utility functions
+'''
+import shutil
+import sys
+import os
+from typing import List
+
+
+def determine_wellknown_cmd(envvar, progname) -> List[str]:
+    maybe_env = os.getenv(envvar)
+    maybe_which = shutil.which(progname)
+    if maybe_env:
+        return maybe_env.split(' ')  # Well-known vars are often meant to be word-split
+    elif maybe_which:
+        return [ maybe_which ]
+    else:
+        sys.exit(f"{progname} not found")