j2k: flatten test cases and testData directory structure

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

j2k/testData/fileOrElement/enum/enumConstantIsNotNull.kt

@@ -0,0 +1,7 @@
+enum class E {
+    FOO
+
+    fun foo() {
+        FOO.toString()
+    }
+}