Do not check cognitive complexity in unit tests

Works around:

> utils/user_test.go:5:1: cognitive complexity 18 of func `TestRuleToRegex` is high (> 15) (gocognit)

Alternatively, we may look for a way to skip this test only.
It doesn't seem complex at all, so it looks like some false-positive.

.golangci.yml

@@ -83,6 +83,7 @@ issues:
     - path: _test\.go
       linters:
         - gocyclo
+        - gocognit
         - errcheck
         - dupl
         - gosec