Add unit tests for new lint

Author: nahuakang

tests/ui/for_loops_over_options.rs

@@ -0,0 +1,31 @@
+#![warn(clippy::for_loops_over_options)]
+
+fn main() {
+    let x = vec![Some(1), Some(2), Some(3)];
+    for n in x {
+        if let Some(n) = n {
+            println!("{}", n);
+        }
+    }
+
+    let y: Vec<Result<i32, i32>> = vec![];
+    for n in y.clone() {
+        if let Ok(n) = n {
+            println!("{}", n);
+        }
+    }
+
+    // This should not trigger the lint
+    for n in y.clone() {
+        if let Ok(n) = n {
+            println!("{}", n);
+        } else {
+            println!("Oops!");
+        }
+    }
+
+    // This should not trigger the lint
+    for n in vec![Some(1), Some(2), Some(3)].iter().flatten() {
+        println!("{}", n);
+    }
+}