Moving from Travis to GitHub Actions about crystal

[ErikSchierboom]

We now have a really nice document and some starting workflows that you can use to help switch to GitHub Actions: https://github.com/exercism/problem-specifications/tree/master/workflows#workflow-templates

[petertseng]

I present below a plausible .github/workflows/tests.yml file that could be used.
The reason I'm not putting it in a PR is that it doesn't really make sense for me to send it as a PR when there are existing build failures on the track (fetch configlet needs to be fixed, test failures need to be fixed). If the tests for the track get fixed, then I would consider sending the PR.

name: Tests
on:
  pull_request:
  push:
    branches:
    - master
    - gha
  schedule:
    # Weekly.
    - cron: "0 0 * * 0"

jobs:
  track-config:
    name: Check track configuration
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
      - name: Fetch
        run: bin/fetch-configlet
      - name: Lint
        run: bin/configlet lint .
  exercises:
    name: Check exercises
    runs-on: ubuntu-latest
    # idea from https://github.com/actions/starter-workflows/blob/master/ci/crystal.yml
    container:
      image: crystallang/crystal
    steps:
      - uses: actions/checkout@v2
      - name: Run tests
        run: make test