Parallelize tests

This aims at simplifying & tweak the tests setup so that they can have their own specific folder, with a specific database file, in order to run them parallelized.

On my machine this allows the tests run to go from:

time meson test
[...]
meson test  22.36s user 8.83s system 20% cpu 2:32.74 total

to:

time meson test
[...]
meson test  24.11s user 12.60s system 68% cpu 53.282 total