test: check for (possibly) missing test coverage

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.