As discussed in #1620, the attribute names for `#[tokio::main]` and
`#[tokio::test]` aren't great. Specifically, they both use
`single_thread` and `multi_thread`, as opposed to names that match the
runtime names: `current_thread` and `threadpool`. This PR changes the
former to the latter.
Fixes#1627.
In the past, it was not possible to choose to use the multi-threaded
tokio `Runtime` in tests, which meant that any test that transitively
used `executor::threadpool::blocking` would fail with
```
'blocking' annotation used from outside the context of a thread pool
```
This patch adds a runtime annotation attribute to `#[tokio::test]` just
like `#[tokio::main]` has, which lets users opt in to the threadpool
runtime over `current_thread` (the default).
Jon Gjengset