REFACTOR: move run-tests and fix the std assert namespace (#9303)

related to the namespace bullet point in
- https://github.com/nushell/nushell/issues/8450

# Description
this was the last module of the standard library with a broken
namespace, this PR takes care of this.

- `run-tests` has been moved to `std/mod.nu`
- `std/testing.nu` has been moved to `std/assert.nu`
- the namespace has been fixed
- `assert` is now called `main` and used in all the other `std assert`
commands
- for `std assert length` and `std assert str contains`, in order not to
shadow the built-in `length` and `str contains` commands, i've used
`alias "core ..." = ...` to (1) define `foo` in `assert.nu` and (2)
still use the builtin `foo` with `core foo` (replace `foo` by `length`
or `str contains`)
  - tests have been fixed accordingly

# User-Facing Changes
one can not use
```
use std "assert equal"
```
anymore because `assert ...` is not exported from `std`.
`std assert` is now a *real* module.

# Tests + Formatting
- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- ⚫ `toolkit test`
- ⚫ `toolkit test stdlib`

# After Submitting
```
$nothing
```

# Notes for reviewers
to test this, i think the easiest is to
- run `toolkit test stdlib` and see all the tests pass
- run `cargo run -- -n` and try `use std assert` => are all the commands
available in scope?

crates/nu-std/src/lib.rs

@@ -23,7 +23,7 @@ pub fn load_standard_library(
             ("help.nu", include_str!("../std/help.nu")),
             ("iter.nu", include_str!("../std/iter.nu")),
             ("log.nu", include_str!("../std/log.nu")),
-            ("testing.nu", include_str!("../std/testing.nu")),
+            ("assert.nu", include_str!("../std/assert.nu")),
             ("xml.nu", include_str!("../std/xml.nu")),
         ];