The existing checks all use a timestamp only 67413 seconds after the
Epoch, which a double can still represent exactly to the nanosecond, so
they do not detect a formatter that goes through floating point. They
also never look at a timestamp before the Epoch, where the fraction has
to be truncated towards minus infinity.
* tests/stat/stat-nanoseconds.sh: Also check %Y, %.Y, %.3Y and %.9Y on a
2026 timestamp and on a timestamp shortly before the Epoch.
Link: https://github.com/uutils/coreutils/pull/13976
Link: https://github.com/coreutils/coreutils/pull/331
0b8fff44cb