macros: Disambiguate the built-in #[test] attribute in macro expansion.

`tokio::test` and related macros now use the absolute path
`::core::prelude::v1::test` to refer to the built-in `test` macro.

This absolute path was introduced in rust-lang/rust#62086.

tokio-macros/src/entry.rs

@@ -142,7 +142,7 @@ fn parse_knobs(
     let header = {
         if is_test {
             quote! {
-                #[test]
+                #[::core::prelude::v1::test]
             }
         } else {
             quote! {}
@@ -334,14 +334,14 @@ pub(crate) mod old {
 
         let result = match runtime {
             Runtime::Threaded => quote! {
-                #[test]
+                #[::core::prelude::v1::test]
                 #(#attrs)*
                 #vis fn #name() #ret {
                     tokio::runtime::Runtime::new().unwrap().block_on(async { #body })
                 }
             },
             Runtime::Basic | Runtime::Auto => quote! {
-                #[test]
+                #[::core::prelude::v1::test]
                 #(#attrs)*
                 #vis fn #name() #ret {
                     tokio::runtime::Builder::new()

tokio/tests/macros_test.rs

@@ -0,0 +1,15 @@
+use tokio::test;
+
+#[test]
+async fn test_macro_can_be_used_via_use() {
+    tokio::spawn(async {
+        assert_eq!(1 + 1, 2);
+    }).await.unwrap();
+}
+
+#[tokio::test]
+async fn test_macro_is_resilient_to_shadowing() {
+    tokio::spawn(async {
+        assert_eq!(1 + 1, 2);
+    }).await.unwrap();
+}