Use meteortesting:mocha to allow headless client tests in CI

Currently this template packages uses `tinytest` which is easy to use and straight forward. However, if we want to add CI we need to enable headless client tests, which tinytest does not support.

My proposed changes:

Use `metertesting:mocha` in combination with npm `chai` and `puppeteer` to run CI tests.
I would propose a PR, incuding the GitHub actions example.

Start by locating the package's current tinytest setup and checking how tests are run in CI. Evaluate the proposed meteortesting:mocha, chai, Puppeteer, and GitHub Actions combination; done means headless client tests run successfully in CI and the template documents the setup.

Written by the indexing model from the issue text.