buf_copy.rs: don't test a trivial io::copy wrapper

[oech3]

Closes #12343

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/printf/printf-surprise is now passing!
Congrats! The gnu test tests/seq/seq-epipe is now passing!
Note: The gnu test tests/env/env-signal-handler was skipped on 'main' but is now failing.