test: check for (possibly) missing test coverage
    
    If the test logs contain lines like:
    
    ```
    ...systemd-resolved[735885]: profiling:/systemd-meson-build/src/shared/libsystemd-shared-250.a.p/base-filesystem.c.gcda:Cannot open
    ```
    
    it means we're possibly missing some coverage since gcov can't write the stats,
    usually due to the sandbox being too restrictive (e.g. ProtectSystem=yes,
    ProtectHome=yes) or the $BUILD_DIR being inaccessible to non-root users.