fix num_proc=1 ci test (#7714)

Author: lhoestq

tests/test_arrow_dataset.py

@@ -1558,7 +1558,7 @@ def test_map_caching_partial_remap(self, in_memory):
                 os.remove(cache_file["filename"])
 
             self._caplog.clear()
-            dset_test2_num_proc = 1
+            dset_test2_num_proc = None
             with dset.map(lambda x: {"foo": "bar"}, num_proc=dset_test2_num_proc) as dset_test2:
                 self.assertEqual(dset_test1_data_files, dset_test2.cache_files)
                 self.assertEqual(len(dset_test2.cache_files), 0 if in_memory else dset_test1_num_proc)
@@ -1569,6 +1569,18 @@ def test_map_caching_partial_remap(self, in_memory):
             for cache_file in dset_test1_data_files[num_files_to_delete:]:
                 os.remove(cache_file["filename"])
 
+            self._caplog.clear()
+            dset_test2_num_proc = 1
+            with dset.map(lambda x: {"foo": "bar"}, num_proc=dset_test2_num_proc) as dset_test2:
+                self.assertEqual(dset_test1_data_files, dset_test2.cache_files)
+                self.assertEqual(len(dset_test2.cache_files), 0 if in_memory else dset_test1_num_proc)
+                self.assertTrue((expected_msg in self._caplog.text) ^ in_memory)
+                self.assertFalse(f"Spawning {dset_test1_num_proc} process" in self._caplog.text)
+                self.assertTrue(f"Spawning {dset_test2_num_proc} process" in self._caplog.text)
+
+            for cache_file in dset_test1_data_files[num_files_to_delete:]:
+                os.remove(cache_file["filename"])
+
             self._caplog.clear()
             dset_test3_num_proc = 3
             with dset.map(lambda x: {"foo": "bar"}, num_proc=dset_test3_num_proc) as dset_test3: