Disallow string downcast for groups and repeats

[ktuite]

Closes #381

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

Unskipping some tests that existed before.

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

It was easy to add this to the downcast checking function!

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?

Shouldn't have a big impact on users unless they're changing their field types a lot. Should help people keep their data cleaner.

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

I don't think so.

Before submitting this PR, please make sure you have:

• run make test-full and confirmed all checks still pass OR confirm CircleCI build passes

[ktuite]

Hi QA!
Here are two forms that represent the change from a group field to a string with the same name as a group:

group downcast.xlsx

group downcast (group is now string).xlsx

I just tried on https://test.getodk.cloud/#/projects/445 (not updated? at v2023.1.) and it is allowed.

Also, here's the message that mentions 'now in different groups or repeats' that it detected but didn't stop:

It should not be allowed in staging.

[dbemke]

Hi. Thank you for the information, it was really helpful to know where the change to string happened. I changed the forms a bit and tried to upload them to Central. I've noticed that if I change the group/repeat field to a string and put them inside a repeat/group I'm still able to upload the one changed to a string. I don't know if it's a valid case - should it also be prevented.

group.downcast.xlsx
group.downcast.group.is.now.string.xlsx