Migrate boxWithJava tests to multi-file framework

compiler/testData/codegen/boxWithJava/varargs.kt

@@ -0,0 +1,31 @@
+// FILE: A.java
+
+public class A {
+    public int foo(int x, String ... args) {
+        return x + args.length;
+    }
+
+    public static String[] ar = new String[] { "a", "b"};
+}
+
+// FILE: test.kt
+
+fun bar(args: Array<String>?): Int {
+    var res  = 0
+
+    if (args != null) {
+        res += A().foo(1, *args)
+    }
+
+    res += A().foo(1, *A.ar)
+
+    return res
+}
+
+fun box(): String {
+    if (bar(null) != 3) return "Fail"
+
+    if (bar(A.ar) != 6) return "Fail"
+
+    return "OK"
+}