Add GitHub Actions workflow.

Author: brocaar

.github/workflows/main.yml

@@ -0,0 +1,9 @@
+name: CI
+on: [push, pull_request]
+jobs:
+  tests:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Run the tests
+        run: make test