dd: move the test-only parse shim into the unit tests

[sylvestre]

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[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)

[codspeed-hq]

Merging this PR will improve performance by 3.92%

⚡ 1 improved benchmark
✅ 42 untouched benchmarks
⏩ 364 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	cksum_crc32b	42.4 ms	40.8 ms	+3.92%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing caret-dd-test-shim (a13709e) with main (f2987b5)²}

Footnotes

1. 364 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. ↩

2. No successful run was found on caret-usage-hint (f2987b5) during the generation of this report, so main (f2987b5) was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

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