j2k: flatten test cases and testData directory structure

Move j2k/test/tests -> j2k/tests, j2k/test/testData -> j2k/testData

j2k/testData/fileOrElement/mutableCollections/Overrides.java

@@ -0,0 +1,16 @@
+package javaApi;
+
+import kotlinApi.KotlinClass;
+
+import java.util.Collection;
+import java.util.List;
+
+public abstract class C extends KotlinClass {
+    public C(int field) {
+        super(field);
+    }
+
+    public List<Object> foo(Collection<String> mutableCollection, Collection<? extends Integer> nullableCollection) {
+        return super.foo(mutableCollection, nullableCollection);
+    }
+}