[data] deflake test_streaming_backpressure_e2e (#51999)

## Why are these changes needed?

Explicitly set RAY_object_spilling_threshold to 1 for this test. So Ray
Core only starts to spill when the object store utilization reaches
100%.


Signed-off-by: Hao Chen <chenh1024@gmail.com>

Author: raulchen

python/ray/data/tests/test_backpressure_e2e.py

@@ -287,9 +287,15 @@ def range_(i):
     assert launched <= 10, launched
 
 
-def test_streaming_backpressure_e2e(restore_data_context):  # noqa: F811
+def test_streaming_backpressure_e2e(
+    shutdown_only, monkeypatch, restore_data_context  # noqa: F811
+):
     # This test case is particularly challenging since there is a large input->output
     # increase in data size: https://github.com/ray-project/ray/issues/34041
+
+    # Increase the Ray Core spilling threshold to 100% to avoid flakiness.
+    monkeypatch.setenv("RAY_object_spilling_threshold", "1")
+
     class TestSlow:
         def __call__(self, df: np.ndarray):
             time.sleep(2)