test: use dom testing library

[davidlj95]

Provides good patterns for testing components. Plus it's not far from what we do rn: componentTestSetup -> render

And it's a way of testing that's framework agnostic. Installing another library would allow same tests to run on another framework. Though with caveats ofc. As all Angular - specific test setup would need a migration. Like the component class to render to start with. But a step towards that no matter what.

https://github.com/testing-library/angular-testing-library