j2k: flatten test cases and testData directory structure

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

j2k/testData/fileOrElement/mutableCollections/Overrides.kt

@@ -0,0 +1,10 @@
+package javaApi
+
+import kotlinApi.KotlinClass
+
+public abstract class C(field: Int) : KotlinClass(field) {
+
+    override fun foo(mutableCollection: MutableCollection<String>, nullableCollection: Collection<out Int>?): MutableList<Any> {
+        return super.foo(mutableCollection, nullableCollection)
+    }
+}