Fix invlet tests

Rewrite the item management of the the invlet test to use an item
unique id hack to make it more robust and functional now that items on
the map are unordered.

Note: this test seems to have been giving false positives since #30603
was merged, since the AUTO_INV_ASSIGN option was not being properly
set.  This only became apparent after implementing the more robust
item management code with unique ids.

This also fixes a few remaing calls in the tests to a removed vehicle
function which still compiled due to implicit conversion of 0 to a pointer.