j2k: flatten test cases and testData directory structure

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

j2k/testData/fileOrElement/nullability/MethodResultInitializesNullableField.kt

@@ -0,0 +1,11 @@
+// ERROR: Unresolved reference: x
+class C {
+    private val string = getString()
+
+    class object {
+
+        fun getString(): String? {
+            return x()
+        }
+    }
+}