Add test on behavior when existing instance is opened for edit

[lognaturel]

Static defaults from the form definition get used for nodes that are omitted from the existing instance.

edit_defaults.xlsx

[lognaturel]

To support this, let's add Scenario method to load an existing instance. This could be an overload of init or something like initWithInstance. The instance will be passed in as an XFormsElement.

[seadowg]

From Slack:

And on testing and responsibilities with editing when it comes to JavaRosa and Scenario: that's bad right now IMO. Collect does way more than I'd expect to "attach" the existing instance when loading a form: https://github.com/getodk/collect/blob/0b52f5115e04b5ef4857ec1e1d58b2a5265e0888/co[…]src/main/java/org/odk/collect/android/tasks/FormLoaderTask.java

To me it feels like JavaRosa could expose a nice way to parse the instance XML into a FormInstance and then there should be an easy path to intializing form entry with a FormDef and that FormInstance. JavaRosa's FormDef is basically both the form "engine" and the immutable definition of the form at the same time though so there's a lot of tangling going on.

To summarize my own thoughts, there's more than just testing here: we need to actually support editing as a feature in JavaRosa. I'd suggest closing this for the moment and waiting to engage with the idea again when we finally get rid of FormLoaderTask in Collect (maybe for getodk/collect#6023).