Merge boxWithJava testData into box, delete BoxWithJava test

compiler/testData/codegen/box/casts/functions/javaTypeIsFunK.kt

@@ -0,0 +1,16 @@
+// FILE: JFun.java
+
+class JFun implements kotlin.jvm.functions.Function0<String> {
+    public String invoke() {
+        return "OK";
+    }
+}
+
+// FILE: test.kt
+
+fun box(): String {
+    val jfun = JFun()
+    val jf = jfun as Any
+    if (jf is Function0<*>) return jfun()
+    else return "Failed: jf is Function0<*>"
+}