j2k: flatten test cases and testData directory structure

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

j2k/testData/fileOrElement/nullability/Overrides.kt

@@ -0,0 +1,31 @@
+open class Base {
+    public open fun foo(s: String?): String? {
+        return ""
+    }
+
+    public open fun bar(s: String?): String? {
+        return if (s != null) s + 1 else null
+    }
+
+    public open fun zoo(o: Any): String {
+        return ""
+    }
+}
+
+trait I {
+    public fun zoo(o: Any?): String?
+}
+
+class C : Base(), I {
+    override fun foo(s: String?): String? {
+        return ""
+    }
+
+    override fun bar(s: String?): String? {
+        return ""
+    }
+
+    override fun zoo(o: Any?): String? {
+        return ""
+    }
+}