private static Optional<DocumentFrequency> makeDocumentFrequency(com.yahoo.language.significance.DocumentFrequency source) {
        return Optional.of(new DocumentFrequency(source.frequency(), source.corpusSize()));
    }
Tests significance term in the search chain.

@author MariusArhaug
container-search/src/test/java/com/yahoo/search/significance/test/SignificanceSearcherTest.java