Add unit tests

[getodk]

Add unit tests

[matthew-white]

We're just going to focus on unit tests for now, not integration tests (like those using Selenium), as integration tests can be challenging to set up. Once we add integration tests, @ggalmazor suggests:

we could try to combine some approval tests and snapshots

This is a really good tool for that: https://github.com/GoogleChrome/puppeteer

[matthew-white]

From @ggalmazor:

https://medium.freecodecamp.org/simple-unit-tests-with-vue-test-utils-and-jest-c384d7abc321

And this is an example on how to use PageObject pattern in combination with Vue component testing https://github.com/just-boris/page-object/blob/master/examples/vue/src/LoginForm.test.js