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-command/tests/commands/do_.rs

@@ -9,6 +9,8 @@ fn capture_errors_works() {
         "#
     ));
 
+    eprintln!("actual.err: {:?}", actual.err);
+
     assert!(actual.err.contains("column_not_found"));
 }
 
@@ -65,7 +67,7 @@ fn ignore_shell_errors_works_for_external_with_semicolon() {
     let actual = nu!(
         cwd: ".", pipeline(
         r#"
-        do -s { fail }; `text`
+        do -s { open asdfasdf.txt }; "text"
         "#
     ));