[MM-61467] Fix errcheck linter issues in post_test.go (#30686)

server/.golangci.yml

@@ -99,7 +99,6 @@ issues:
         channels/app/platform/session.go|\
         channels/app/platform/status.go|\
         channels/app/platform/web_hub_test.go|\
-        channels/app/post_test.go|\
         channels/app/slashcommands/command_test.go|\
         channels/app/slashcommands/helper_test.go|\
         channels/app/team.go|\