fix: add arg count for cp

[LoricAndre]

Fixes #6689

[sylvestre]

could you please add a test to make sure we don't regress in the future
thanks

[LoricAndre]

I didn't know how to robustly match the multiline error in test_cp_no_file, but I think this is enough to make sure that the missing argument error is caught by clap.
However, this does change the error message compared to GNU cp when no argument is provided:

GNU cp :

cp: missing file operand
Try 'cp --help' for more information.

uu-cp:

error: the following required arguments were not provided:
  <paths>...

Usage: target/debug/coreutils cp [OPTION]... [-T] SOURCE DEST
       target/debug/coreutils cp [OPTION]... SOURCE... DIRECTORY
       target/debug/coreutils cp [OPTION]... -t DIRECTORY SOURCE...

For more information, try '--help'.

[LoricAndre]

The CI seems to have timed out, could someone restart it please ?

[sylvestre]

sure, done

[LoricAndre]

Is the error message discrepancy an issue ? I'm not familiar with how close this projects aims to stay to GNU coreutils in that regard.

[sylvestre]

nope, we are often doing better :)