Failures against master because of OneOf changes

The OneOf validator was changed in https://github.com/ckan/ckan/pull/5060 to replace the formencode one, but the output when there is a validation error is slightly different:

```
New: u"Value must be one of [u'bactrian', u'hybrid', u'f2hybrid', u'snowwhite', u'black']"

Old "Value must be one of: bactrian; hybrid; f2hybrid; snowwhite; black (not 'rocker')"
```

This causes the validation tests errors for sure, I'm not sure about the form functional tests but they could also be related

Start with the validation tests mentioned in the issue, then inspect the related form functional tests to identify assertions affected by the OneOf message change. Compare their expected output with the old and new examples, and consider the work complete when the relevant test failures are resolved consistently.

Written by the indexing model from the issue text.