roottest/root/io/newstl should preserves its data files. #20313
Description
As discussed https://github.com/root-project/root/pull/19709#discussion_r2470311313 the pr #19709 remove (in practice) the automatic ability for the roottest/root/io/newstl:run test to preserve the ROOT files produced by each 'known' version of ROOT. The test should be upgraded to preserve, at least once per ROOT release, those data files and to use those preserved files (at least in some [CI] builds).