Add missing test Java files

java/ql/test/library-tests/pattern-switch/dfg/GuardTest.java

@@ -0,0 +1,30 @@
+public class GuardTest {
+
+  public static void sink(String s) { }
+  public static boolean isSafe(String s) { return s.length() < 10; }
+
+  public static void test(Object o) {
+
+    switch (o) {
+
+      case String s:
+        sink(s);
+        break;
+      default:
+        break;
+
+    }
+
+    switch (o) {
+
+      case String s when isSafe(s):
+        sink(s);
+        break;
+      default:
+        break;
+
+    }
+
+  }
+
+}