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
