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/fs.rs

@@ -0,0 +1,20 @@
+#![warn(rust_2018_idioms)]
+#![cfg(feature = "full")]
+
+use tokio::fs;
+use tokio_test::assert_ok;
+
+#[tokio::test]
+async fn path_read_write() {
+    let temp = tempdir();
+    let dir = temp.path();
+
+    assert_ok!(fs::write(dir.join("bar"), b"bytes").await);
+    let out = assert_ok!(fs::read(dir.join("bar")).await);
+
+    assert_eq!(out, b"bytes");
+}
+
+fn tempdir() -> tempfile::TempDir {
+    tempfile::tempdir().unwrap()
+}