j2k: flatten test cases and testData directory structure

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

j2k/testData/fileOrElement/nullability/OverrideWithInheritanceLoop.kt

@@ -0,0 +1,11 @@
+// ERROR: There's a cycle in the inheritance hierarchy for this type
+// ERROR: There's a cycle in the inheritance hierarchy for this type
+open class A : B() {
+    public open fun foo(s: String) {
+    }
+}
+
+open class B : A() {
+    public open fun foo(s: String) {
+    }
+}