HPCC-11881 Roxie assert reading file::ip::x::y::z

We can live without the size for the last part in a multipart file. Safer than
trying to fix the IFileDescriptor to give us the size, and probably more
efficient too.

Also fix the setuptext test so that it can run (and reveal this issue).

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #6210 from richardkchapman/roxie-file-assert

HPCC-11881 Roxie assert reading file::ip::x::y::z

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>