add xt_mode option

Just adjusting filename isn't enough because the test used '-Mblib' which won't work running out of xt.

This might be a bit screwy if someone uses xt mode and then also does [ExtraTests], but trying to account for that gets trickier.  (look for blib and fallback to lib?)   

That could just be documented, which I haven't done.  ("Don't use xt_mode if you use ExtraTests")