test: add isomtime field to fix test isolation in test_extract.py

Add isomtime field to test data default dictionary so tests work
regardless of borg_compat.version state. The code uses either mtime
or isomtime depending on borg version, so providing both ensures
tests pass when run in isolation or alongside other tests.

Author: m3nu

tests/unit/test_extract.py

@@ -31,6 +31,7 @@ def prepare_borg(mocker, borg_json_output):
     "linktarget": "",
     "flags": None,
     "mtime": "2022-05-13T14:33:57.305797",
+    "isomtime": "2022-05-13T14:33:57.305797",
     "size": 0,
 }