Fixed depth test for 3D texture test

Change-Id: I80a735a614a132319903298594f1990c49efdee4
Reviewed-on: https://swiftshader-review.googlesource.com/2140
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>