ci(GitHub): add testing workflow

* Added .github/workflows/testing.yml w/ jobs test & benchmark.

Author: userzimmermann

.github/workflows/testing.yml

@@ -0,0 +1,76 @@
+name: Test & benchmark
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    
+    steps:
+    - uses: actions/checkout@v5
+      with:
+        fetch-depth: 0
+    
+    - name: Setup Go environment
+      uses: actions/setup-go@v6
+      with:
+        go-version: 1.25.1
+    
+    - name: Run tests
+      run: |
+          go mod tidy
+          go test -v ./...
+
+  benchmark:
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v5
+      with:
+        fetch-depth: 0
+
+    - name: Setup Go environment
+      uses: actions/setup-go@v6
+      with:
+        go-version: 1.25.1
+
+    - name: Install benchstat
+      run: go install golang.org/x/perf/cmd/benchstat@latest
+
+    - name: Run benchmark on base branch
+      if: github.event_name == 'pull_request'
+      run: |
+          git checkout ${{ github.event.pull_request.base.sha }}
+          go mod tidy
+          go test -v -run ^$ -bench . -count ${{ vars.GO_BENCHMARK_COUNT }} -benchmem > base-bench.txt
+          git checkout ${{ github.event.pull_request.head.sha }}
+
+    - name: Run benchmark on HEAD commit
+      run: |
+          go mod tidy
+          go test -v -run ^$ -bench . -count ${{ vars.GO_BENCHMARK_COUNT }} -benchmem > head-bench.txt
+
+    - name: Compare benchmarks
+      if: github.event_name == 'pull_request'
+      run: |
+          cat > benchmark-comment.md << EOF
+          ## Benchmark comparison w/ base branch
+
+          \`\`\`
+          $(benchstat base-bench.txt head-bench.txt)
+          \`\`\`
+          EOF
+
+    - name: Comment PR w/ benchmark comparison
+      if: github.event_name == 'pull_request'
+      uses: actions/github-script@v8
+      with:
+        script: |
+            github.rest.issues.createComment({
+                issue_number: context.issue.number,
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                body: require('fs').readFileSync('benchmark-comment.md', 'utf8')
+            });