t/lib-httpd: make CGI test helpers concurrency-safe #28960

Job	Run time
config	8s
fuzz smoke test	1m 11s
win build	12m 5s
static-analysis	43s
win+Meson build	12m 5s
sparse	3m 53s
documentation	12m 5s
osx-clang (macos-14)	2m 20s
osx-reftable (macos-14)	1m 50s
osx-meson (macos-14)	3m 30s
osx-gcc (macos-14)	12m 5s
rust-analysis	12m 5s
linux-reftable (ubuntu:rolling)	12m 5s
fedora-breaking-changes-meson (fedora:latest)	12m 5s
linux-sha256 (ubuntu:rolling)	12m 5s
linux-meson (ubuntu:rolling)	12m 5s
debian-11 (debian:11)	12m 5s
linux-asan-ubsan (ubuntu:rolling)	12m 5s
linux-reftable-leaks (ubuntu:rolling)	12m 5s
linux-breaking-changes (ubuntu:rolling)	12m 5s
almalinux-8 (almalinux:8)	12m 5s
linux-musl-meson (alpine:latest)	12m 5s
linux-TEST-vars (ubuntu:20.04)	12m 5s
win+VS build	0s
linux-leaks (ubuntu:rolling)	12m 5s
linux32 (i386/ubuntu:20.04)	12m 5s
win+VS test	0s
win+Meson test	0s
win test	0s
	3h 51m 5s

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