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>
1a8143f721