testEngine: rename mock methods to be less generic
testEngine: rename mock methods to be less generic

The engine mock methods did mocking specific to a particular
xapian-bridge method, so alter their names to reflect that.

[endlessm/eos-sdk#3703]

endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib