Minor change to `ArcanistInfrastructureTestCase::testLibraryMap`

Summary: Modify `ArcanistInfrastructureTestCase::testLibraryMap` to use `phutil_get_current_library_name()` instead of hard-coding the library name.

Test Plan: See D9844.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9846