There was a bug in the "prepend_vector_object" test but it was masked
by us not printing failures. (The bug was that we were adding three
elements to the "objects" vector and then checking that another
vector called "more_objects" indeed had three elements. Oops!)
This allows you to do things like:
vector.insert_before_matching(value, [](auto& entry) {
return value < entry;
});
Basically it scans until it finds an element that matches the condition
callback and then inserts the new value before the matching element.
