The videotestsrc is not build if the adder plugin is disabled. This is a
copy/paste error introduced in Commit 3de86b2b9725 ("docs: port plugins to
explicit sources").
Fix the check to actually test the 'videotestsrc' option.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8399>
They are part of gst_dep already and we have to make sure to always have
gst_dep. The order in dependencies matters, because it is also the order
in which Meson will set -I args. We want gstreamer's config.h to take
precedence over glib's private config.h when it's a subproject.
While at it, remove useless fallback args for gmodule/gio dependencies,
only gstreamer core needs it.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2031>
