Add preload / Store tests #362
Description
Once esm compatible, restart #183 and investigate whether we can easily test preload scripts. As a concrete example, when updating mobx #361 / #159 I had to QA by clicking around app until I got to a mobx store, which would reveal issues in makeObservable / observable property setup. Having specs that exercise the very basic store logic (property setting, updating, watching) would exercise that logic fine.
I believe I punted on this last time because of esm, testing libraries, etc. Bundling with esm upgrade work should clear the way.