Tests for entity-updating forms modifying dataset properties

[ktuite]

Closes getodk/central#516

Forms that update entities vs. creating entities don't get parsed any differently at upload time. The effects these forms have on dataset properties are the same as entity-creation forms. This PR adds a few tests of this behavior.

I think these tests will be more important to have in place when we get back to this issue: getodk/central#518 (Restrict submissions to the entity actions listed in the form) because that will involve code changes around dataset parsing and form uploading.

What has been done to verify that this works as intended?

Why is this the best possible solution? Were any other approaches considered?

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

Does this change require updates to the API documentation? If so, please update docs/api.md as part of this PR.

Before submitting this PR, please make sure you have:

• run make test-full and confirmed all checks still pass OR confirm CircleCI build passes
• verified that any code from external sources are properly credited in comments or that everything is internally sourced