GnuTests: fix hardcoded pathes to GNU executables

[oech3]

Replace hardcoded pathes calling GNU's executables

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/dd/misc. tests/dd/misc is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/df/over-mount-device. tests/df/over-mount-device is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/tail/follow-name (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/cat/splice is no longer failing!
Congrats! The gnu test tests/cut/cut is no longer failing!
Congrats! The gnu test tests/cut/mb-non-utf8 is no longer failing!
Congrats! The gnu test tests/dd/partial-write is no longer failing!
Congrats! The gnu test tests/expand/mb is no longer failing!
Congrats! The gnu test tests/ls/stat-free-symlinks is no longer failing!
Congrats! The gnu test tests/misc/close-stdout is no longer failing!
Congrats! The gnu test tests/mktemp/write-error is no longer failing!
Congrats! The gnu test tests/mv/dir2dir is no longer failing!
Congrats! The gnu test tests/mv/mv-exchange is no longer failing!
Congrats! The gnu test tests/nl/multibyte is no longer failing!
Congrats! The gnu test tests/od/od-float is no longer failing!
Congrats! The gnu test tests/od/od-j is no longer failing!
Congrats! The gnu test tests/ptx/ptx-overrun is no longer failing!
Congrats! The gnu test tests/sort/sort-merge-fdlimit is no longer failing!
Congrats! The gnu test tests/unexpand/mb is no longer failing!
Note: The gnu test tests/expand/bounded-memory is now being skipped but was previously passing.
Note: The gnu test tests/dd/fail-ftruncate-fstat was skipped on 'main' but is now failing.