search: tests for the elasticsearch walker
- Adds tests for the elasticsearch AST walker. These tests make sure the AST produced from the user's query by the invenio-query parser is transformed correctly into an ElasticSearch-DSL query.
Signed-off-by: Marios Kogias <ekogias@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Reviewed-by: Javier Martin Montull <javier.martin.montull@cern.ch>