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)

tests/commands/main_test.go

@@ -0,0 +1,11 @@
+package tests
+
+import (
+	"testing"
+
+	"go.uber.org/goleak"
+)
+
+func TestMain(m *testing.M) {
+	goleak.VerifyTestMain(m)
+}