FIR: Add test on ambiguous vararg

Fix is in the two previous commits

compiler/testData/codegen/box/javaInterop/ambiguousJavaVararg.kt

@@ -0,0 +1,28 @@
+// FILE: JavaInterface.java
+
+public interface JavaInterface {
+    String foo(Object... obj);
+
+    String foo(String... str);
+}
+
+// FILE: JavaClass.java
+
+public class JavaClass implements JavaInterface {
+    public String foo(Object... obj) {
+        return "FAIL";
+    }
+
+    public String foo(String... str) {
+        return "OK";
+    }
+}
+
+// FILE: test.kt
+// TARGET_BACKEND: JVM
+
+class KotlinClass : JavaClass()
+
+fun box(): String {
+    return KotlinClass().foo("alpha", "omega")
+}