Tests: add a test to make sure that function can't use mutable variable (#14314)

@sholderbach suggested that we need to have a test for a function can't
use mutable variable.

https://github.com/nushell/nushell/pull/14311#issuecomment-2470035194

So this pr is going to add a case for it.

---------

Co-authored-by: Stefan Holderbach <sholderbach@users.noreply.github.com>

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

@@ -141,3 +141,10 @@ fn mut_raw_string() {
     let actual = nu!(r#"mut x = r#'abc'#; $x"#);
     assert_eq!(actual.out, "abc");
 }
+
+#[test]
+fn def_should_not_mutate_mut() {
+    let actual = nu!("mut a = 3; def foo [] { $a = 4}");
+    assert!(actual.err.contains("capture of mutable variable"));
+    assert!(!actual.status.success())
+}