rt: fix basic_scheduler notification bug (#1861)

The "global executor" thread-local is to track where to spawn new tasks,
**not** which scheduler is active on the current thread. This fixes a
bug with scheduling tasks on the basic_scheduler by tracking the
currently active basic_scheduler with a dedicated thread-local variable.

Fixes: #1851

tokio/tests/rt_common.rs

@@ -198,6 +198,21 @@ rt_test! {
         }
     }
 
+    #[test]
+    fn spawn_await_chain() {
+        let mut rt = rt();
+
+        let out = rt.block_on(async {
+            assert_ok!(tokio::spawn(async {
+                assert_ok!(tokio::spawn(async {
+                    "hello"
+                }).await)
+            }).await)
+        });
+
+        assert_eq!(out, "hello");
+    }
+
     #[test]
     fn outstanding_tasks_dropped() {
         let mut rt = rt();