Add missing manpages

[oech3]

1. Add missing manpages (for part of hashsum).
2. Test existense of man1/md5sum.1 instead of whoami.1.

[codspeed-hq]

CodSpeed Performance Report

Merging #8721 will not alter performance

_{Comparing oech3:man-hasusum (ccad088) with main (5287738)}

Summary

✅ 45 untouched
⏩ 1 skipped¹

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/stdbuf (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)