* [lang] Introduce [ink::test] macro

* [core] Migrate some tests to use [ink::test] macro

* Address comments

* Apply cargo fmt

* [lang] Ensure generated code behaves as if test fn has no return value

* [examples] Migrate tests to use [ink::test] macro

* [core] Revert core using [ink::test]

* [lang] Use all available parameters

* [lang] Remove unused imports

* [lang] Remove implicit Ok(())

* [core] Apply cargo fmt

* [lang] Make it clearer where ItemFn comes from

* Apply cargo fmt