Wiki: Started a page on how to write testcases for GDB

[Joel Brobecker]

> Thanks.
> It would be nice to have this as part of gdbint.texinfo as well.

I think we should put the info in either document, but not both, or
we would be duplicating the info.  I chose the Wiki because it's very
easy to update and maintain.  To be honest, I don't think I will have
the courage to make fixes if it means having to go through the normal
review process.

-- 
Joel