meson: add build_tests option

Similar to the corresponding option in gtk: https://gitlab.gnome.org/GNOME/gtk/-/blob/26b250503d73c364e6dbe3627473e990ab109fe0/meson_options.txt#L122-125

I have two reasons for this:

• speed up the build when used as a subproject
• allow running the build in a noexec sandbox, because currently building the tests requires executing gdk-pixbuf-pixdata. This is causing issues with github actions in gtk-rs: https://github.com/gtk-rs/gtk-rs-core/runs/6384870101?check_suite_focus=true#step:4:12750