Move unit test runner to standard library (#8850)

Move test runner to standard library.
Originated from #8819 

# After Submitting

I'll update the documentation about testing:
http://www.nushell.sh/book/testing.html

---------

Co-authored-by: Mate Farkas <Mate.Farkas@oneidentity.com>

crates/nu-std/lib/mod.nu

@@ -1,12 +1,12 @@
 # std.nu, `used` to load all standard library components
 
-export use assert *
 export use dirs *
 export-env {
     use dirs *
 }
 export use help *
 export use log *
+export use testing *
 export use xml *
 
 # Add the given paths to the PATH.