Add unit test runner with coverage

[bvotteler]

After initially mostly only having stored data with very little transformations, we are starting to use Squid with more and more complex logic to track metrics, volumes, etc. It will be beneficial to be able to easily add unit tests.

Therefore, we want to add a test runner setup with code coverage to be run in github actions for pull requests.
A good initial test case to add could be this one: #41

For code coverage, we need to exclude the generated code folders populated by squids scripts (via yarn command code:gen and gen:types:*).

Optional: Consider adding a coverage rule that needs to be passed that roughly resembles this:

• If coverage is above 90% automatically pass
• otherwise, expect coverage percentage to have increased.
  If this is too complex / time consuming to add as part of this issue, create a follow up one instead.