Fix parallel testing on ci

This merge request fixes parallel CI builds in the most simple way.

For Linux host, /dev/shm is used to store the test database and prefix to speedup tests execution.

However, on the Gitlab CI, the same machine may run several builds in parallel which means that one run might overwrite data from another one.

In order to avoid that two solutions are possible:

• Use the database and prefix "in place" (i.e. in the same folder as the code)
• Use the various Gitlab provided environment variables to build a path in /dev/shm to ensure each build has its data in its own directory

The first solution was chosen has it is the easiest to put in place.