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

.github/workflows/ci.yml

@@ -129,7 +129,7 @@ jobs:
         run: cargo install --path . --locked --no-default-features
 
       - name: Standard library tests
-        run: nu -c 'use std testing; testing run-tests --path crates/nu-std'
+        run: nu -c 'use crates/nu-std/testing.nu; testing run-tests --path crates/nu-std'
 
       - name: Setup Python
         uses: actions/setup-python@v5