Add test for functional interfaces

java/ql/test/library-tests/functional-interfaces/Test.java

@@ -0,0 +1,29 @@
+public class Test {
+
+  interface Functional {
+    int f();
+  }
+
+  class Concrete implements Functional {
+    public int f() { return 0; }
+  }
+
+  interface FunctionalWithDefaults {
+    int f();
+
+    default int g() { return 1; }
+  }
+
+  interface NotFunctional {
+    default int g() { return 1; }
+  }
+
+  interface FunctionalWithObjectMethods {
+    int f();
+
+    String toString();
+
+    int hashCode();
+  }
+
+}

java/ql/test/library-tests/functional-interfaces/test.expected

@@ -0,0 +1,3 @@
+| Test.java:3:13:3:22 | Functional |
+| Test.java:11:13:11:34 | FunctionalWithDefaults |
+| Test.java:21:13:21:39 | FunctionalWithObjectMethods |

java/ql/test/library-tests/functional-interfaces/test.ql

@@ -0,0 +1,3 @@
+import java
+
+select any(FunctionalInterface fi | fi.fromSource())