hwdb-test: pass an explit path to systemd-hwdb

hwdb-test: pass an explit path to systemd-hwdb

https://github.com/systemd/systemd/pull/19316 failed with:
[1065/1670] Linking target systemd-hwdb
--- command ---
14:28:29 /root/src/test/hwdb-test.sh
--- stdout ---
./systemd-hwdb does not exist, please build first

I'm not sure what is going on here… In principle meson says that tests may be
called from any directory, but in practice is was always the build directory.
So far we were relying on systemd-hwdb being present in '.', and this worked.
Either way, it's nicer to pass the exact path, so let's do that.