Add test coverage for client and component libraries

[jbrunton]

Right now there are two 'core' packages:

• packages/metrics – This is the code for computing flow metrics, forecasts and chart data.
• apps/api – This is the code for the application API, which is responsible for syncing and storing data and configuration options.

There are also packages focused on web client concerns:

• packages/components – Common components for displaying issues and reports.
• packages/charts – Charts for displaying flow metrics.
• apps/client – The web client itself.

The core packages (and also packages/lib) have decent test coverage, because they are crucial to syncing and storing data, and to computing that data correctly. It's therefore essential that they are tested to ensure data are correct.

The client packages should ideally be tested, but not to the same degree. For now, they aren't tested, and while that could lead to occasional bugs in the UX, it won't lead to incorrect data.

For this reason, Sonar Cloud is configured to compute coverage metrics only for the core packages (and lib). Coverage metrics are therefore incomplete for the project, but provide a meaningful summary of coverage of the core packages.