Fix jshint tests

Summary:
The linter's behavior was changed in https://github.com/jshint/jshint/issues/3444: "warnings" are no longer counted for maxerr.

Updating the test to match...

Test Plan: arc unit src/lint/linter/__tests__/ArcanistJSHintLinterTestCase.php with a recent-ish (2.13.6) jshint.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25355