Tests: Test that a world can generate with item links

[alwaysintreble]

What is this fixing or adding?

Moves Item Link creation out of main; there's likely a better place but this is what made the most sense to me. Also adds a unit test for generating with full item link replacements. Since most worlds generate filler randomly, that filler won't actually get replaced, so unsure on how to best add actual tests beyond "doesn't crash".