Tests for wfGetIP() follow up r89407

Tests for wfGetIP() follow up r89407

* wfGetIP() now support resetting its internal static variable. Thanks to
 Platonides which introduced this trick with r92960.
* Various tests for $_SERVER['REMOTE_ADDR'] and $wgCommandLineMode.

TODO:

- implements tests for XFF headers.

TEST PLAN:

$ ./phpunit.php --filter wfGetIP --testdox
PHPUnit 3.5.14 by Sebastian Bergmann.

wfGetIP
 [x] Get loopback address when in command line
 [x] Get from server remote addr
 [x] Lack of remote addr throw an exception
$