md5sum: remove some stderr_is("") in the tests

[cakebaker]

No description provided.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/csplit/csplit-heap is now passing!
Note: The gnu test tests/env/env-signal-handler was skipped on 'main' but is now failing.