Fix config for test_api.py.
Fix config for test_api.py.

Summary:
A seemingly wrong entry for MSG_PARTS_DIRECTORY in tests/config.cfg was causing
test_headers_for_message to fail when it couldn't find the data on
disk.

Test Plan: ./runtests

Reviewers: kav-ya

Reviewed By: kav-ya

Differential Revision: https://review.inboxapp.com/D10

nylas/sync-engine,wakermahmud/sync-engine,rmasters/inbox,rmasters/inbox,Eagles2F/sync-engine,jobscore/sync-engine,closeio/nylas,EthanBlackburn/sync-engine,PriviPK/privipk-sync-engine,Eagles2F/sync-engine,Eagles2F/sync-engine,nylas/sync-engine,ErinCall/sync-engine,Eagles2F/sync-engine,gale320/sync-engine,ErinCall/sync-engine,gale320/sync-engine,closeio/nylas,gale320/sync-engine,rmasters/inbox,wakermahmud/sync-engine,gale320/sync-engine,EthanBlackburn/sync-engine,jobscore/sync-engine,ErinCall/sync-engine,wakermahmud/sync-engine,wakermahmud/sync-engine,ErinCall/sync-engine,jobscore/sync-engine,PriviPK/privipk-sync-engine,nylas/sync-engine,jobscore/sync-engine,PriviPK/privipk-sync-engine,closeio/nylas,Eagles2F/sync-engine,PriviPK/privipk-sync-engine,ErinCall/sync-engine,EthanBlackburn/sync-engine,wakermahmud/sync-engine,gale320/sync-engine,PriviPK/privipk-sync-engine,EthanBlackburn/sync-engine,rmasters/inbox,EthanBlackburn/sync-engine,nylas/sync-engine,closeio/nylas