numfmt: cap IEC precision at 3 decimals

[pocopepe]

Fixes #11926

Added effective_precision = precision.min(3) for iec units in consider_suffix to match gnu's rounding behaviour. Tests added for both the IEC cap and that SI is unaffected.
Verified against GNU 9.4.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/timeout/timeout-group. tests/timeout/timeout-group is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/symlink (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/cp/link-heap is now being skipped but was previously passing.

[codspeed-hq]

Unable to generate the performance report

There was an internal error while processing the run's data. We're working on fixing the issue. Feel free to contact us on Discord or at [email protected] if the issue persists.