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)

tests/shell/pipeline/commands/external.rs

@@ -85,7 +85,7 @@ fn execute_binary_in_string() {
 
 #[test]
 fn single_quote_dollar_external() {
-    let actual = nu!("let author = 'JT'; ^echo $'foo=($author)'");
+    let actual = nu!("let author = 'JT'; nu --testbin cococo $'foo=($author)'");
 
     assert_eq!(actual.out, "foo=JT");
 }
@@ -493,18 +493,16 @@ mod external_command_arguments {
         )
     }
 
-    #[cfg(not(windows))]
     #[test]
     fn semicolons_are_sanitized_before_passing_to_subshell() {
-        let actual = nu!("^echo \"a;b\"");
+        let actual = nu!("nu --testbin cococo \"a;b\"");
 
         assert_eq!(actual.out, "a;b");
     }
 
-    #[cfg(not(windows))]
     #[test]
     fn ampersands_are_sanitized_before_passing_to_subshell() {
-        let actual = nu!("^echo \"a&b\"");
+        let actual = nu!("nu --testbin cococo \"a&b\"");
 
         assert_eq!(actual.out, "a&b");
     }