Running tests with Meteor 3+

When running the tests with Meteor 3+ installed I get huge package conflicts mostly due to `practicalmeteor:*` packages involved.
Since there is no CI on this project I can not determine how the core team runs this in backward compat mode.

Updating them to use `meteortesting:mocha` works only if on 3.0.0 because 2.x throws ` Error: Cannot find module 'fibers'`
From my end there are the following things that this repo needs:

- update to 3.0 compatibility for testing (`meteortesting:mocha`)
- add a GitHub actions CI with Matrix test for Meteor 1.7, 2.3 and 3.x
- update README

Start by reviewing the existing test setup and README, then reproduce the reported conflicts with Meteor 2.x and 3.x. Check how meteortesting:mocha interacts with the supported versions. Done means tests work across the stated Meteor versions, GitHub Actions runs the requested matrix, and the README documents the process.

Written by the indexing model from the issue text.