Check WEBKITOUTPUTDIR when configuring fonts in WebKitTestRunner

Look at non-standard build directory (WEBKITOUTPUT env variable)
when loading fonts for WebKitTestRunner, as is already done in
Gtk's WTK and Efl's DumpRenderTree implementations.
There is a bug when you use WEBKITOUTPUTDIR to build webkit into a
custom build directory and then try to run layout tests using that build.
If you try to do that before this patch, WTR fails completely rendering
fonts.

Signed-off-by: Nick Diego Yamane <nick.yamane@openbossa.org>

Reviewed by: Rafael Brandão