java: add test for aliasing

found by triage

Author: yoff

java/ql/test/query-tests/ThreadSafe/ThreadSafe.expected

@@ -1,3 +1,4 @@
+| examples/Alias.java:16:13:16:13 | y | This field access (publicly accessible via $@) is not protected by any monitor, but the class is annotated as @ThreadSafe. | examples/Alias.java:16:13:16:13 | y | this expression |
 | examples/C.java:14:9:14:14 | this.y | This field access (publicly accessible via $@) is not protected by any monitor, but the class is annotated as @ThreadSafe. | examples/C.java:14:9:14:14 | this.y | this expression |
 | examples/C.java:15:9:15:14 | this.y | This field access (publicly accessible via $@) is not protected by any monitor, but the class is annotated as @ThreadSafe. | examples/C.java:15:9:15:14 | this.y | this expression |
 | examples/C.java:16:9:16:14 | this.y | This field access (publicly accessible via $@) is not protected by any monitor, but the class is annotated as @ThreadSafe. | examples/C.java:16:9:16:14 | this.y | this expression |

java/ql/test/query-tests/ThreadSafe/examples/Alias.java

@@ -0,0 +1,21 @@
+package examples;
+
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
+
+@ThreadSafe
+public class Alias {
+    private int y;
+
+    private final ReentrantLock lock = new ReentrantLock();
+
+    public void notMismatch() {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            y = 42; // $ SPURIOUS: Alert
+        } finally {
+            this.lock.unlock();
+        }
+    }
+}