A set of fixes for stderr redirect (#7219)

# Description

This is a set of fixes to `err>` to make it work a bit more predictably.

I've also revised the tests, which accidentally tested the wrong thing
for redirection, but should be more correct now.

# User-Facing Changes

_(List of all changes that impact the user experience here. This helps
us keep track of breaking changes.)_

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-cli/src/repl.rs

@@ -528,7 +528,7 @@ pub fn evaluate_repl(
             Err(err) => {
                 let message = err.to_string();
                 if !message.contains("duration") {
-                    println!("Error: {:?}", err);
+                    eprintln!("Error: {:?}", err);
                     // TODO: Identify possible error cases where a hard failure is preferable
                     // Ignoring and reporting could hide bigger problems
                     // e.g. https://github.com/nushell/nushell/issues/6452