t/lib-httpd: make CGI test helpers concurrency-safe #28959

Job	Run time
config	7s
fuzz smoke test	2m 2s
win+Meson build	18s
win build	19s
rust-analysis	19s
static-analysis	23s
documentation	19s
sparse	5s
osx-clang (macos-14)	19s
osx-reftable (macos-14)	18s
osx-gcc (macos-14)	19s
osx-meson (macos-14)	19s
linux32 (i386/ubuntu:20.04)	19s
linux-reftable (ubuntu:rolling)	19s
linux-musl-meson (alpine:latest)	19s
almalinux-8 (almalinux:8)	19s
win+VS build	0s
linux-asan-ubsan (ubuntu:rolling)	19s
linux-breaking-changes (ubuntu:rolling)	19s
debian-11 (debian:11)	19s
fedora-breaking-changes-meson (fedora:latest)	19s
linux-sha256 (ubuntu:rolling)	19s
linux-TEST-vars (ubuntu:20.04)	19s
linux-reftable-leaks (ubuntu:rolling)	19s
linux-meson (ubuntu:rolling)	19s
linux-leaks (ubuntu:rolling)	19s
win+VS test	0s
win+Meson test	-1s
win test	0s
	9m 13s

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