gh-68141: Guard test_mmap.LargeMmapTests.* with a timeout

[arhadthedev]

Some file systems don't support sparse files required by test_mmap to work with huge sizes in theory only, without extremely long and likely unsuccessful initialization of disc space.

Currently test_mmap.LargeMmapTests._make_test_file() assumes that sparse files are unsupported on Windows and macOS but always supported on Linux. However, filesystems like ZFS available on Linux break this assumption. As a result, we should rely on difference in duration of initialization between a single sector for sparse files and many gigabytes for non-sparse files.

Since SHORT_TIMEOUT can be changed with --timeout, we should put a really short bar enough for 4KB page to be written even in the worst case of process management and antivirus scan (like min(SHORT_TIMEOUT, 5.0)).

• Issue: test_mmap uses cruel and unusual amounts of disk space #68141

[arhadthedev]

Looking for a way to force-abort a worker in ThreadPoolExecutor on timeout.