Regression tests for VM "customElements"

[elliotBraem]

This is in reference to the customElements feature defined in the near-social-vm, code block below:

https://github.com/NearSocial/VM/blob/6047c6a9b96f3de14e600c1d2b96c432bbb76dd4/src/lib/vm/vm.js#L524-L530

Tests

Custom Elements

• [ ] should throw error if a custom element is referenced from code, but not defined
• [ ] should render custom element when defined
• [ ] should successfully pass attributes to custom element
• [ ] should successfully pass children to custom element