Regression(r138603): Caused 2 webkitpy tests to fail
https://bugs.webkit.org/show_bug.cgi?id=105913

Reviewed by Kentaro Hara.

Fix 2 webkitpy tests that started to fail after
r138603.

* Scripts/webkitpy/tool/bot/irc_command_unittest.py:
(IRCCommandTest._sheriff_test_data_url): Make sure the
returned test file URL is valid no matter what the
current working directory is.
* Scripts/webkitpy/tool/bot/ircbot_unittest.py:
(IRCBotTest.test_help): Add "sheriffs" command to the
list of available commands.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@138617 268f45cc-cd09-0410-ab3c-d52691b4dbfc