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>

src/test_bins.rs

@@ -70,7 +70,7 @@ pub fn echo_env_mixed() {
 }
 
 /// Cross platform echo using println!()
-/// Example: nu --testbin echo a b c
+/// Example: nu --testbin cococo a b c
 /// a b c
 pub fn cococo() {
     let args: Vec<String> = args();