Add module tests

[danielgamage]

should probably test a few uses of the module on every push:

• minimal react example
• minimal web component example
• web component auto-register example

just need to ensure they render, but maybe testing for the existence of the exclusion element would be useful? a visual diffing test might be ideal but maybe more trouble to set up than it's worth?

[mikabusante]

Added some extremely basic specs in #9, basically just testing if the element is created without an error.
Still looking for a better way to test the contents of the shadow DOM 🔍