j2k: flatten test cases and testData directory structure

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

j2k/testData/fileOrElement/nullability/autoNotNull.kt

@@ -0,0 +1,8 @@
+class A {
+    fun foo(s: String?): Int {
+        if (s != null) {
+            return s.length()
+        }
+        return -1
+    }
+}