.github: add CI tests

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>

.github/workflows/test.yml

@@ -0,0 +1,18 @@
+name: tests
+
+on: [push]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+
+      - name: Set up Go
+        uses: actions/setup-go@v3
+        with:
+          go-version: "1.20"
+
+      - name: Test
+        run: go test -v ./...