ARROW-10073: [Python] Don't rely on dict item order in test_parquet_nested_storage

Instead, refer to fields by name

Closes #8248 from bkietz/10073-Test-test-parquet-nested-

Authored-by: Benjamin Kietzman <bengilgit@gmail.com>
Signed-off-by: Krisztián Szűcs <szucs.krisztian@gmail.com>
diff --git a/python/pyarrow/tests/test_extension_type.py b/python/pyarrow/tests/test_extension_type.py
index 8bc1a30..3757b8c 100644
--- a/python/pyarrow/tests/test_extension_type.py
+++ b/python/pyarrow/tests/test_extension_type.py
@@ -511,8 +511,8 @@
     pq.write_table(orig_table, filename)
 
     table = pq.read_table(filename)
-    assert table.column(0).type == mystruct_array.type
-    assert table.column(1).type == mylist_array.type
+    assert table.column('structs').type == mystruct_array.type
+    assert table.column('lists').type == mylist_array.type
     assert table == orig_table