Refactor tests (using cococo instead of ^echo) (#11479)

- related PR: #11478 

# Description

Now we can use `nu --testbin cococo` instead of `^echo` to echo messages
to stdout in tests.

But `nu` treats parameters as its own flags when parameter starts with
`-`. So `^echo --foo='bar'` still use `^echo`.

# User-Facing Changes

(none)

# Tests + Formatting

- [x] `cargo fmt --all -- --check` to check standard code formatting
(`cargo fmt --all` applies these changes)
- [x] `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used`
to check that you're using the standard code style
- [x] `cargo test --workspace` to check that all tests pass (on Windows
make sure to [enable developer
mode](https://learn.microsoft.com/en-us/windows/apps/get-started/developer-mode-features-and-debugging))
- [x] `cargo run -- -c "use std testing; testing run-tests --path
crates/nu-std"` to run the tests for the standard library


# After Submitting

(none)

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

@@ -197,7 +197,7 @@ fn def_wrapped_with_block() {
 #[test]
 fn def_wrapped_from_module() {
     let actual = nu!(r#"module spam {
-            export def --wrapped my-echo [...rest] { ^echo ...$rest }
+            export def --wrapped my-echo [...rest] { nu --testbin cococo ...$rest }
         }
 
         use spam