Force gc to avoid non-deterministic failure

Change-Id: I164e7a554a2ac53e205eb67da9edc42f0ba4cc38

python/pyarrow/tests/test_convert_builtin.py

@@ -47,6 +47,10 @@ class TestConvertList(unittest.TestCase):
 
     def test_garbage_collection(self):
         import gc
+
+        # Force the cyclic garbage collector to run
+        gc.collect()
+
         bytes_before = pyarrow.total_allocated_bytes()
         pyarrow.from_pylist([1, None, 3, None])
         gc.collect()