Add tests and docs for tests

[liammulh]

EDIT: We have Vitest set up and working. #245 adds the ability to create E2E tests with Cypress. The original comments in this issue are stale. See #25 (comment) for up-to-date status.

[liammulh]

Unit tests are added in #123.

[liammulh]

The ability to add integration tests is also done in #123.

[gwhitney]

Is there an example integration test? What are the relevant source files? Thanks.