numfmt: fix empty delimiter and whitespace handling

[ChrisDryden]

Theres still around 61 failing tests in numfmt, around 14 of them relate to how empty delimiters and whitespace is handled. This PR adds a bunch of integration tests based on the GNU integration tests and adds the logic that decides whether the number of whitespaces is valid or invalid.

The high level gist is that the amount of whitespaces defaults to 1 but is should be equal to the length of the delimiter option if it is explicitly provided.

These are all of the gnu test cases fixed by this pr:

  - delim-2
  - unit-sep-6, unit-sep-7, unit-sep-8
  - unit-sep-11, unit-sep-12, unit-sep-15 
  - unit-sep-21
  - suf-19, suf-21, suf-22, suf-23, suf-24, suf-25
  - lcl-suf-1, lcl-suf-2, lcl-suf-8

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/shuf/shuf-reservoir (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/sort/sort-stale-thread-mem (passes in this run but fails in the 'main' branch)

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/overlay-headers (fails in this run but passes in the 'main' branch)

[codspeed-hq]

Merging this PR will not alter performance

✅ 284 untouched benchmarks
⏩ 38 skipped benchmarks¹

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_{Comparing ChrisDryden:fix-numfmt-empty-delimiter (0ac6458) with main (ec7e81e)}

Footnotes

1. 38 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[sylvestre]

needs a rebase, sorry

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/pr/bounded-memory is no longer failing!

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/misc/sync is no longer failing!

[github-actions]

GNU testsuite comparison:

Note: The gnu test tests/unexpand/bounded-memory is now being skipped but was previously passing.