Add functional test for datastore functionality

[Debashis9012]

Currently we don't have any functional tests to validate datastore features.
We should cover it for all language plugins.
Reference

[sriv]

Some datastore specs are available here: https://github.com/getgauge/gauge-tests/blob/master/specs/advanced_topics/data_store.spec

Is there any extra feature that needs to be added to the tests?