tests/mkfifo: added a test to check mkfifo permission denied error for code coverage

[asder8215]

This PR introduces a test case that provides code coverage for the following lines in uu/mkfifo/src/mkfifo.rs:

        // Explicitly set the permissions to ignore umask
        if let Err(e) = fs::set_permissions(&f, fs::Permissions::from_mode(mode)) {
            return Err(USimpleError::new(
                1,
                translate!("mkfifo-error-cannot-set-permissions", "path" => f.quote(), "error" => e),
            ));
        }

With this, it should complete #9059 and we should have 100% code coverage for mkfifo.rs.

[github-actions]

GNU testsuite comparison:

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

[codspeed-hq]

CodSpeed Performance Report

Merging #9586 will improve performances by 2.13%

_{Comparing asder8215:test/mkfifo-cov (09f7c5d) with main (7c62885)}

Summary

⚡ 1 improvement
✅ 126 untouched
⏩ 6 skipped¹

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	tsort_input_parsing_heavy[5000]	83.8 ms	82.1 ms	+2.13%

Footnotes

1. 6 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. ↩

[cakebaker]

Thanks!

[sylvestre]

kudos!