env: add regression tests for non-UTF-8 command names and directory execution

[krosci]

Execution failures on non-UTF-8 binary paths and directories previously reported generic unknown errors instead of the underlying OS error message. This pull request adds regression tests verifying that non-UTF-8 program names, directory invocations, and paths with trailing slashes report the correct error message and exit code.

Resolves #12805
Resolves #12778.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/retry (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/misc/tty-eof (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/unexpand/bounded-memory is now passing!