test: goroutine leak detector (#45)

- Add goroutine leak detector to help avoid Goroutine leaks by Uber:
https://github.com/uber-go/goleak
- By using this package, I found an issue in frecency:
- Close global DB connection to fix goroutine leak in tests
(https://github.com/versenilvis/IRIS/commit/831f86ffdf01124da537c149559fa41f37500e1e)

root/main_test.go

@@ -0,0 +1,21 @@
+package root
+
+import (
+	"os"
+	"testing"
+
+	"go.uber.org/goleak"
+	"github.com/versenilvis/iris/internal/scoring"
+)
+
+func TestMain(m *testing.M) {
+	code := m.Run()
+	scoring.CloseGlobalFrecencyStore()
+	if code == 0 {
+		if err := goleak.Find(); err != nil {
+			_, _ = os.Stderr.WriteString("goleak: " + err.Error() + "\n")
+			os.Exit(1)
+		}
+	}
+	os.Exit(code)
+}