Implement tests

[daun]

• Implement unit tests
• Implement e2e tests
• A lot can be re-used from core tests

[hirasso]

For the e2e tests, I'd like to try a solution similar to what I did in hirasso/scrollmirror:

Setup an Astro site for all the test fixtures that can also be used as a development site. ScrollPlugin's source could then directly be imported into the astro site, simplifying iterative development.

[hirasso]

• Setup Tests #91

[daun]

Guess this was already implemented via #97 :)