hide std testing (#11331)

follow-up to
- https://github.com/nushell/nushell/pull/11151

> **Important**
> land only between 0.89 and 0.90

# Description
this PR hides the `std testing` module from the outside.
- moves `nu-std/std/testing.nu` to `nu-std/testing.nu`
- removes the module from the standard library list of modules to parse
- fixes `toolkit.nu` and the CI

# User-Facing Changes
`std testing` won't be part of the standard library anymore.
# Tests + Formatting

# After Submitting

crates/nu-std/src/lib.rs

@@ -18,7 +18,6 @@ pub fn load_standard_library(
 
         let mut std_files = vec![
             ("mod.nu", include_str!("../std/mod.nu")),
-            ("testing.nu", include_str!("../std/testing.nu")),
             ("dirs.nu", include_str!("../std/dirs.nu")),
             ("dt.nu", include_str!("../std/dt.nu")),
             ("help.nu", include_str!("../std/help.nu")),