clean cp tests (#12202)

# Description
There are lots of duplicate test for `cp`, it's because we once have
`old-cp` command.

Today `old-cp` is removed, so there is no need to keep these tests.

crates/nu-command/tests/commands/mod.rs

@@ -12,7 +12,6 @@ mod config_env_default;
 mod config_nu_default;
 mod continue_;
 mod conversions;
-mod cp;
 #[cfg(feature = "sqlite")]
 mod database;
 mod date;