Force timeit to not capture stdout (#12465)

# Description
Fixes:  #11996

After this change `let t = timeit ^ls` will list current directory to
stdout.
```
❯ let t = timeit ^ls
CODE_OF_CONDUCT.md      Cargo.lock              Cross.toml              README.md               aaa                     benches                 devdocs                 here11                  scripts                 target                  toolkit.nu              wix
CONTRIBUTING.md         Cargo.toml              LICENSE                 a.txt                   assets                  crates                  docker                  rust-toolchain.toml     src                     tests                   typos.toml
```

If user don't want such behavior, he can redirect the stdout to `std
null-stream` easily
```
> use std
> let t = timeit { ^ls o> (std null-device) }
```

# User-Facing Changes
NaN

# Tests + Formatting
Done

# After Submitting
Nan

---------

Co-authored-by: Ian Manske <ian.manske@pm.me>

crates/nu-command/tests/commands/debug/mod.rs

@@ -0,0 +1 @@
+mod timeit;

crates/nu-command/tests/commands/debug/timeit.rs

@@ -0,0 +1,14 @@
+use nu_test_support::nu;
+
+#[test]
+fn timeit_show_stdout() {
+    let actual = nu!("let t = timeit nu --testbin cococo abcdefg");
+    assert_eq!(actual.out, "abcdefg");
+}
+
+#[test]
+fn timeit_show_stderr() {
+    let actual = nu!(" with-env {FOO: bar, FOO2: baz} { let t = timeit { nu --testbin echo_env_mixed out-err FOO FOO2 } }");
+    assert!(actual.out.contains("bar"));
+    assert!(actual.err.contains("baz"));
+}

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

@@ -113,6 +113,7 @@ mod ucp;
 #[cfg(unix)]
 mod ulimit;
 
+mod debug;
 mod umkdir;
 mod uname;
 mod uniq;