Add drop tracking version of must_not_suspend ref test

Author: eholk

src/test/ui/lint/must_not_suspend/ref-drop-tracking.rs

@@ -0,0 +1,30 @@
+// edition:2018
+// compile-flags: -Zdrop-tracking
+#![feature(must_not_suspend)]
+#![deny(must_not_suspend)]
+
+#[must_not_suspend = "You gotta use Umm's, ya know?"]
+struct Umm {
+    i: i64
+}
+
+struct Bar {
+    u: Umm,
+}
+
+async fn other() {}
+
+impl Bar {
+    async fn uhoh(&mut self) {
+        let guard = &mut self.u; //~ ERROR `Umm` held across
+
+        other().await;
+
+        *guard = Umm {
+            i: 2
+        }
+    }
+}
+
+fn main() {
+}