Add Unit tests GitHub Actions workflow

The README advertised a Unit tests badge but no such workflow existed, and the test package was never run in CI. Runs build, vet, and 'make test' on pushes to main and on PRs.

Author: kisst

.github/workflows/test.yml

@@ -0,0 +1,28 @@
+# Unit test workflow.
+name: Unit tests
+
+# Run unit tests and vet on pushes to main and on every pull request.
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+
+permissions:
+  contents: read
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@c85c95e3d7251135ab7dc9ce3241c5835cc595a9 # v3.5.3
+      - uses: actions/setup-go@93397bea11091df50f3d7e59dc26a7711a8bcfbe # v4.1.0
+        with:
+          go-version-file: 'go.mod'
+          cache: true
+      - name: Build
+        run: go build ./...
+      - name: Vet
+        run: go vet ./...
+      - name: Unit tests
+        run: make test