[Train] Monkeypatch environment variables in test_json (ray-project…

…#21260)

If we use `os.environ` to set environment variables in tests, then our tests become coupled. By using `monkeypatch`, we can safely set environment variables while ensuring our tests remain decoupled. 

For more information, see the [monkeypatching documentation](https://docs.pytest.org/en/6.2.x/monkeypatch.html#monkeypatching-environment-variables).

python/ray/train/tests/test_callbacks.py

@@ -59,10 +59,10 @@ def on_shutdown(self, worker_group: WorkerGroup,
 @pytest.mark.parametrize("workers_to_log", [0, None, [0, 1]])
 @pytest.mark.parametrize("detailed", [False, True])
 @pytest.mark.parametrize("filename", [None, "my_own_filename.json"])
-def test_json(ray_start_4_cpus, make_temp_dir, workers_to_log, detailed,
-              filename):
+def test_json(monkeypatch, ray_start_4_cpus, make_temp_dir, workers_to_log,
+              detailed, filename):
     if detailed:
-        os.environ[ENABLE_DETAILED_AUTOFILLED_METRICS_ENV] = "1"
+        monkeypatch.setenv(ENABLE_DETAILED_AUTOFILLED_METRICS_ENV, "1")
 
     config = TestConfig()
 
@@ -119,9 +119,6 @@ def train_func():
             all(not any(key in worker for key in DETAILED_AUTOFILLED_KEYS)
                 for worker in element) for element in log)
 
-    os.environ.pop(ENABLE_DETAILED_AUTOFILLED_METRICS_ENV, 0)
-    assert ENABLE_DETAILED_AUTOFILLED_METRICS_ENV not in os.environ
-
 
 def _validate_tbx_result(events_dir):
     events_file = list(glob.glob(f"{events_dir}/events*"))[0]