test(tokenstore): pin the warn-writer default by descriptor, not pointer

Under 'go test -json' (Go 1.26, what gotestsum runs) the testing package replaces the os.Stderr variable after package init to attribute output to tests, so the init-captured default no longer compares equal to os.Stderr even though it is the process's real stderr. Compare the underlying descriptor instead; io.Discard or a buffer still fails the check.