Make tests optional using a meson option

This PR adds a meson option to disable tests. Tests require the installation of some python modules that are useless to the end user once fprintd has been built. In distros like Slackware where a simple build script is distributed and the end user build the software in his own pc, this would mean adding a lot of dependencies that are mostly useless to the end user, since the build has already been validated by the buildscript maintainer.