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/tests/test_dirs.nu

@@ -1,8 +1,5 @@
 use std assert
-use std "assert length"
-use std "assert equal"
-use std "assert not equal"
-use std "assert error"
+use std assert
 use std log
 
 # A couple of nuances to understand when testing module that exports environment:

crates/nu-std/tests/test_setup_teardown.nu

@@ -1,6 +1,5 @@
 use std log
-use std "assert"
-use std "assert skip"
+use std assert
 
 export def setup [] {
     log debug "Setup is running"

crates/nu-std/tests/test_std.nu

@@ -1,7 +1,7 @@
 use std
 
 export def test_path_add [] {
-    use std "assert equal"
+    use std assert
 
     let path_name = if "PATH" in $env { "PATH" } else { "Path" }
 

crates/nu-std/tests/test_xml.nu

@@ -1,7 +1,7 @@
 use std xml xaccess
 use std xml xupdate
 use std xml xinsert
-use std "assert equal"
+use std assert
 
 export def setup [] {
     {sample_xml: ('