Minor optimization for unit test

Summary: I noticed that PhabricatorLibraryTestCase::testMethodVisibility takes a long time to execute (up to 90 seconds on my laptop). Looking at the profile it seems that most (54.5%) of the execution time is spent in PhutilLibraryTestCase::getVisibility. Rewriting PhutilLibraryTestCase::testMethodVisibility to use a bitmask instead of strings seems to provide a significant performance improvement (on my local machine, this test now takes 45 seconds, see the profile).

Test Plan: arc unit

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: argp, Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12654