Update gitlab pipeline to make it faster

To reduce the building time for the tests, a new docker image needs to be introduced. This will help make things simpler by reducing the calls for the GitLab CI/CD pipelines.

Steps to take to make the pipeline faster:

• Remove the three separate jobs and merge them in a single tests job.
• Change image for pipeline from python-based image to the docker-obm-database image, that has the latest SpatiaLite version in it.
• Use pre-built PostGIS image for postgis_tests as a service.