j2k: flatten test cases and testData directory structure

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

j2k/testData/fileOrElement/nullability/NullableMethodDotAccess.kt

@@ -0,0 +1,9 @@
+class C {
+    fun getString(b: Boolean): String? {
+        return if (b) "a" else null
+    }
+
+    fun foo(): Int {
+        return getString(true)!!.length()
+    }
+}