Use preloads for instanceID in test forms

There are [various test forms](https://github.com/getodk/central-backend/search?q=concat%28%27uuid%3A%27%2C+uuid%28%29%29) that use calculates to set `instanceID`. This is problematic for clients because calculates can be reevaluated without warning and so the `instanceID` is not stable on the client side.

It doesn't matter for Central but sometimes these test forms get used as examples or starting points in unexpected contexts. Let's change those to what `pyxform` outputs and is generally the expected binding for `instanceID`:

```xml

```