cat,comm,expand,unexpand: remove is_dir() pre-checks to let read() report errors

[ChrisDryden]

This PR is based on the investigation of the read-error gnu test failures. Even though we correctly identify that the test is trying to use a directory we are failing the test because the error coming from the read function is different. This led me down the path to discover that we actually do not need to do additional stat checks to check if we are trying to read a directory because that is handled by read() and it is the expectation that we rely on the read() checks to provide the error message.

Every utility in this PR already had integration tests for validating that the error message matches the GNU error message and I have made them more exact by also validating error code and making it stderr_is instead of stderr_contains.

There will probably be two more PR's basically the same as this one for the other utilities

[ChrisDryden]

Hmm I have to figure out what to do for windows here, this is not defined behavior compared to GNU

[codspeed-hq]

Merging this PR will degrade performance by 4.75%

❌ 2 regressed benchmarks
✅ 282 untouched benchmarks
⏩ 38 skipped benchmarks¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	unexpand_many_lines[100000]	189.7 ms	199.2 ms	-4.75%
❌	Simulation	unexpand_large_file[10]	397.7 ms	417.6 ms	-4.75%

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_{Comparing ChrisDryden:fix-read-errors-simple (7509656) with main (8e59663)}

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. ↩

[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)

[ChrisDryden]

It looks like the fix here is to make the is_dir checks a windows only thing instead of removing it entirely