add echo_env_mixed testbin to reduce windows only tests (#11172)

# Description
We have seen some test cases which requires to output message to both
stdout and stderr, especially in redirection scenario.

This pr is going to introduce a new echo_env_mixed testbin, so we can
have less tests which only runs on windows.

# User-Facing Changes
NaN

# Tests + Formatting
NaN

# After Submitting
NaN

src/main.rs

@@ -228,6 +228,7 @@ fn main() -> Result<()> {
         match testbin.item.as_str() {
             "echo_env" => test_bins::echo_env(true),
             "echo_env_stderr" => test_bins::echo_env(false),
+            "echo_env_mixed" => test_bins::echo_env_mixed(),
             "cococo" => test_bins::cococo(),
             "meow" => test_bins::meow(),
             "meowb" => test_bins::meowb(),

src/test_bins.rs

@@ -12,16 +12,51 @@ use std::io::{self, BufRead, Read, Write};
 pub fn echo_env(to_stdout: bool) {
     let args = args();
     for arg in args {
-        if let Ok(v) = std::env::var(arg) {
-            if to_stdout {
-                println!("{v}");
-            } else {
-                eprintln!("{v}");
-            }
+        echo_one_env(&arg, to_stdout)
+    }
+}
+
+fn echo_one_env(arg: &str, to_stdout: bool) {
+    if let Ok(v) = std::env::var(arg) {
+        if to_stdout {
+            println!("{v}");
+        } else {
+            eprintln!("{v}");
         }
     }
 }
 
+/// Mix echo of env keys from input
+/// Example:
+///     * nu --testbin echo_env_mixed out-err FOO BAR
+///     * nu --testbin echo_env_mixed err-out FOO BAR
+/// If it's not present, panic instead
+pub fn echo_env_mixed() {
+    let args = args();
+    let args = &args[1..];
+
+    if args.len() != 3 {
+        panic!(
+            r#"Usage examples:
+* nu --testbin echo_env_mixed out-err FOO BAR
+* nu --testbin echo_env_mixed err-out FOO BAR"#
+        )
+    }
+    match args[0].as_str() {
+        "out-err" => {
+            let (out_arg, err_arg) = (&args[1], &args[2]);
+            echo_one_env(out_arg, true);
+            echo_one_env(err_arg, false);
+        }
+        "err-out" => {
+            let (err_arg, out_arg) = (&args[1], &args[2]);
+            echo_one_env(err_arg, false);
+            echo_one_env(out_arg, true);
+        }
+        _ => panic!("The mixed type must be `out_err`, `err_out`"),
+    }
+}
+
 /// Cross platform echo using println!()
 /// Example: nu --testbin echo a b c
 /// a b c