unexpand: fix +0 and /0 handling, add integration tests

[ChrisDryden]

This is based on the feedback here: #9265 I did a bunch of fuzzing and wanted to add all of the test cases from the previous rounds of feedback to the integ tests. This also addresses one of the edge cases related to parsing +0 and /0

[github-actions]

GNU testsuite comparison:

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

[codspeed-hq]

CodSpeed Performance Report

Merging this PR will improve performance by 5.5%

_{Comparing ChrisDryden:unexpand-fix (a0a85c2) with main (00f77cc)}

Summary

⚡ 3 improved benchmarks
✅ 139 untouched benchmarks
⏩ 180 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	unexpand_large_file[10]	624.9 ms	592.4 ms	+5.5%
⚡	Simulation	unexpand_many_lines[100000]	298.1 ms	282.6 ms	+5.49%
⚡	Simulation	cp_large_file[16]	382.7 µs	369.8 µs	+3.48%

Footnotes

1. 180 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. ↩

[github-actions]

GNU testsuite comparison:

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

[sylvestre]

and it even improved the perf, well done!