platform tests: misc. minor cleanups

- remove unused global / import
- use is_linux and is_darwin
- rename darwin acl test method

src/borg/testsuite/platform_darwin.py

@@ -20,7 +20,7 @@ def set_acl(path, acl, numeric_ids=False):
 
 
 @skipif_acls_not_working
-def test_access_acl():
+def test_extended_acl():
     file = tempfile.NamedTemporaryFile()
     assert get_acl(file.name) == {}
     set_acl(