Summary:
as titled, this will prevent the error that was printed because
test_names was evaluated before db_test was built.

Test Plan:
verified below command works and no longer prints errors:

  $ make release -j32

verified below command still finds the right tests:

  $ make J=32 parallel_check

Reviewers: igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D54117