This fixes the unusual case where the `interactive: false` property had
no effect, which would make it impossible to plan missions against UI
elements that were overflown by many flights (such as the front line).
As an added bonus, it looks a bit nicer.
This impacts the test in an odd way, but the cure for that is probably
rewriting the test to not use a mock now that we've figured out how to
do that.
Fixes https://github.com/dcs-liberation/dcs_liberation/issues/3295.
