Support testing for shared functionality in SpatiaLite and PostGIS database usage

Status

Until now the exposure-lib has had the functionality for spatialite and postgis separate, and the testing for it has been separate. The problem is that maintaining separate code-bases for these two setups does not make sense if the functionality is shared.

Proposal

Reorganize the tests in the exposure-lib so that it allows for testing shared functions for both databases. This can be done either through unittest through an abstract setup or directly through pytest. Will try pytest first.

cc: @ds - @laurens - @pdlmora fyi: @chengzhi - @gislars