This post assumes that you know how the Google App Engine Datastore basically works, and how to perform local unit testing of your code on the dev_server provided in the GAE SDK.
There is a massive gaping hole in the GAE SDK, both in terms of functionality and documentation (of course if there is some documentation on the matter please post it in the comments) in regards to the population of your local datastore (which is persisted to a file) for testing. Why does this matter? For integration, or acceptance testing. Not all testing my Google Overlords wants to be done in a memory only datastore, or within the one process/JVM/however Eclipse runs my dev_server and my JUnit tests.
Even if you setup your test code to point to the same file that your dev_server is reading from, your application wont see your entities. To say it's a frustrating problem is an understatement. Turns out there is a combination of fields that have to be set in your test code for the underlying datastore code to populate the file in such a way to get this to work
Rather than repeat the solution, it can be on the ever helpful Stack Overflow
This solution was found though a lot of pain, trial and error. I hope someone gets a use out of this post to prevent them agonising over the amount of blood lost from when their head hit the desk screaming "why Google why!!!!"