t/lib-httpd: make CGI test helpers concurrency-safe#2171

mmontalbo wants to merge 3 commits into

gitgitgadget:mastergitgitgadget/git:masterfrom

mmontalbo:mm/lib-httpd-cgi-safe-protommontalbo/git:mm/lib-httpd-cgi-safe-protoCopy head branch name to clipboard