During calculation of discontinuous space in a scratch buffer file we need to take into account the pages from the end of the file that are neither allocated nor marked as free
When calculating actively used bytes of scratch file do not add up the sizes but number of requested pages.
If we cannot allocate from the end of scratch buffer file but the number of bytes in use allows to allocate more space then create a new file.
Adjusting tests for multiple scratch buffers usage - both green now.