test: detect OS consistently using platform.system()

test/functional/test_framework/util.py

@@ -13,8 +13,8 @@ import json
 import logging
 import os
 import pathlib
+import platform
 import re
-import sys
 import time
 
 from . import coverage
@@ -414,12 +414,12 @@ def get_temp_default_datadir(temp_dir: pathlib.Path) -> tuple[dict, pathlib.Path
     """Return os-specific environment variables that can be set to make the
     GetDefaultDataDir() function return a datadir path under the provided
     temp_dir, as well as the complete path it would return."""
-    if sys.platform == "win32":
+    if platform.system() == "Windows":
         env = dict(APPDATA=str(temp_dir))
         datadir = temp_dir / "Bitcoin"
     else:
         env = dict(HOME=str(temp_dir))
-        if sys.platform == "darwin":
+        if platform.system() == "Darwin":
             datadir = temp_dir / "Library/Application Support/Bitcoin"
         else:
             datadir = temp_dir / ".bitcoin"