Add a few more tests.

This adds tests for trying to acquire a write lock when a read lock is
held, and trying to acuire a read lock when a write lock is held.

tests/test.rs

@@ -31,3 +31,35 @@ fn double_write_lock() {
 
     drop(g0);
 }
+
+#[test]
+fn read_and_write_lock() {
+    let dir = tempdir().unwrap();
+    let path = dir.path().join("lockfile");
+
+    let l0 = RwLock::new(File::create(&path).unwrap());
+    let mut l1 = RwLock::new(File::open(path).unwrap());
+
+    let g0 = l0.try_read().unwrap();
+
+    let err = l1.try_write().unwrap_err();
+    assert!(matches!(err.kind(), ErrorKind::WouldBlock));
+
+    drop(g0);
+}
+
+#[test]
+fn write_and_read_lock() {
+    let dir = tempdir().unwrap();
+    let path = dir.path().join("lockfile");
+
+    let mut l0 = RwLock::new(File::create(&path).unwrap());
+    let l1 = RwLock::new(File::open(path).unwrap());
+
+    let g0 = l0.try_write().unwrap();
+
+    let err = l1.try_read().unwrap_err();
+    assert!(matches!(err.kind(), ErrorKind::WouldBlock));
+
+    drop(g0);
+}