[Testing] Improve testing

[jeroenptrs]

These files need testing:

Execution

• handleProxy.ts obsolete with new library
• index.ts split attachHandlers in separate file to make testable, the rest doesn't need any tests
• registerScriptHandler.ts (do this in [Execution] registerScriptHandler needs to be more naïve #20)
• spawn.ts

Registry

• index.ts split checkRegistry into sizable chunks [done in feat: store hashes to check file changes #12]

Micro

• the whole library :)

Shared

• none

Registry UI

• test API with something like superagent