add regression test for allow-by-default must_not_suspend

Author: guswynn

src/test/ui/lint/must_not_suspend/issue-89562.rs

@@ -0,0 +1,19 @@
+// edition:2018
+// run-pass
+
+use std::sync::Mutex;
+
+// Copied from the issue. Allow-by-default for now, so run-pass
+pub async fn foo() {
+    let foo = Mutex::new(1);
+    let lock = foo.lock().unwrap();
+
+    // Prevent mutex lock being held across `.await` point.
+    drop(lock);
+
+    bar().await;
+}
+
+async fn bar() {}
+
+fn main() {}