Minor optimization for unit test
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 [[https://secure.phabricator.com/xhprof/profile/PHID-FILE-da7lssvefh6u47cmzmfl/ | 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 [[https://secure.phabricator.com/xhprof/profile/PHID-FILE-n6frqec5yg66tzrck3p5/ | 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

julkiewicz/libphutil,julkiewicz/libphutil,julkiewicz/libphutil