Attached patch makes it possible to enable testmatic via the .fcproj file, i.e., assumption is that testing setup is defined as a project of its own. I think this makes sense as one often wants to define also storage dir so that tests do not risk their "production" data.
I'm still running massive autogame testing with autotools made builds on all branches. I would want to switch that to use meson based builds in main, and likely S3_2. For that goal, functionality similar to autotools --enable-testmatic would be useful.