.github/workflows: switch from Travis CI to GitHub Actions

So long, and thanks for all the builds!

.github/workflows/test.yml

@@ -0,0 +1,10 @@
+name: Go tests
+on: [push, pull_request]
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/setup-go@v2
+        with: { go-version: 1.x }
+      - uses: actions/checkout@v2
+      - run: go test -race ./...