macros: Use more consistent runtime names (#1628)

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.

build-tests/tests/fail/macros_invalid_input.rs

@@ -6,7 +6,7 @@ fn main_is_not_async() {}
 #[tokio::main(foo)]
 async fn main_attr_has_unknown_args() {}
 
-#[tokio::main(multi_thread::bar)]
+#[tokio::main(threadpool::bar)]
 async fn main_attr_has_path_args() {}
 
 #[tokio::test]