Page-cache residency tests: size fixtures past read_ahead_kb and print timing

A Linux test checks that an open path issues madvise/fadvise WILLNEED: it asserts the middle of an index region becomes resident with the advice and stays mostly non-resident without it. That can flake on another machine, because the kernel's own readahead/read-around (bounded by the device's read_ahead_kb) may fill the region anyway. Guards that made it robust: read and print read_ahead_kb, and make the probed middle range several times larger than readahead plus a margin. Prefer an exact 'bytes advised' counter as the pass/fail check (rounding start down and end up to pages like the code does, with a fixture whose size isn't page-aligned). Keep major-fault counts as information only. For a timed residency poll, print when the threshold was reached, or the residency at timeout, so a slow VM can be told apart from missing advice.