Improve testing for building SortFields in SortBuilders

[cbuescher]

This is a follow up issue as discussed in #17205. We have added randomized roundtrip testing for SortBuilders (toXContent -> fromXContent) and serialization tests in AbstractSortBuilderTests, but currently we do no very thourough testing of the SortField that is emitted when calling the sort builders build() method. Most of this is covered by other integration tests, but we should investigate if we can improve the assertions made on the SortFields, while keeping the tests simple.
One of the challanges here is that most sort builders create SortFields of SortField.Type.CUSTOM, providing their own FieldComparatorSource, which in turn wraps most of the builders options and is difficult to access from tests.