Fix generate_hiddenapi_lists_test.py

This change fixes the following tests which were broken by a previous
change (change id I5e3c2f8d14524d59dd65d26ce5196efcc15d6d8e):
* test_assign
* test_parse_and_merge_csv

Bug: 177317659
Test: tools/hiddenapi/generate_hiddenapi_lists_test.py
Change-Id: If29c65794e258a3871269a0214a10980613542e6