Test: add race detector

ref: https://golang.org/doc/articles/race_detector.html

.github/workflows/test.yml

@@ -34,4 +34,4 @@ jobs:
         uses: actions/checkout@v2
 
       - name: Test
-        run: go test -timeout 1h -v ./...
+        run: go test -v -race -timeout 1h ./...