numfmt:fix issue #11653 & correct test cases

[THEMIScale]

Description

Fix the argument parsing behavior of numfmt to fully align with GNU coreutils for negative number inputs.

• Remove allow_negative_numbers to match error message for negative numbers without --
• Add dedicated test cases for issue numfmt: accepts -N negative numbers as positional args where GNU rejects them as invalid options #11653
• Fix the incorrect test_should_not_round_floats test by adding -- (required by GNU for negative values)

Testing

• All new and existing numfmt tests pass
• Behavior now matches numfmt implementation exactly
• No breaking changes to other utilities

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/printf/printf-surprise is now passing!

[cakebaker]

Thanks for your PR!