GRAILS-9162 - improve consistency of testing grails.validation.ValidationErrors

This changes fixes GrailsUnitTestMixin to configure ValidationErrors.getAt to return a String instead of a FieldError which allows GrailsUnitTestMixin tests to behave more like GrailsUnitTestCase tests.
Merge branch 'master' of github.com:grails/grails-core
fix failing tests