testsuite: Fix Windows platform test

Previously we used platform.system() and while this worked fine (e.g.
returned `Windows`, as expected) locally under both msys and MingW64
Python distributions, it inexplicably returned `MINGW64_NT-10.0`
under MingW64 Python on CI. It seems os.name is more reliable so we now
use that instead..

testsuite/driver/testutil.py

@@ -87,8 +87,8 @@ def testing_metrics():
 # are forced to just copy instead.
 #
 # We define the following function to make this magic more
-# explicit/discoverable. You are enouraged to use it instead of os.symlink.
-if platform.system() == 'Windows' and os.getenv('FORCE_SYMLINKS') == None:
+# explicit/discoverable. You are encouraged to use it instead of os.symlink.
+if os.name == 'nt' and os.getenv('FORCE_SYMLINKS') == None:
     def link_or_copy_file(src: Path, dst: Path):
         shutil.copyfile(str(src), str(dst))
 else: