t/lib-httpd: make CGI test helpers concurrency-safe #28962

Job	Run time
config	8s
fuzz smoke test	2m 1s
win build	5m 36s
documentation	7m 30s
win+Meson build	4m 10s
rust-analysis	1m 46s
sparse	3m 42s
osx-meson (macos-14)	15m 12s
osx-gcc (macos-14)	19m 4s
win+VS build	0s
osx-reftable (macos-14)	19m 40s
osx-clang (macos-14)	5m 41s
static-analysis	3m 47s
linux-TEST-vars (ubuntu:20.04)	16m 44s
linux-breaking-changes (ubuntu:rolling)	18m 4s
linux-reftable-leaks (ubuntu:rolling)	14m 7s
almalinux-8 (almalinux:8)	13m 54s
linux-sha256 (ubuntu:rolling)	17m 41s
linux-asan-ubsan (ubuntu:rolling)	16m 19s
debian-11 (debian:11)	14m 26s
fedora-breaking-changes-meson (fedora:latest)	16m 32s
linux-musl-meson (alpine:latest)	11m 22s
linux-meson (ubuntu:rolling)	16m 26s
linux-reftable (ubuntu:rolling)	11m 52s
linux-leaks (ubuntu:rolling)	12m 41s
linux32 (i386/ubuntu:20.04)	10m 57s
win+VS test	0s
win+Meson test (8)	9m 26s
win+Meson test (6)	14m 18s
win+Meson test (9)	9m 3s
win+Meson test (2)	7m 46s
win+Meson test (1)	8m 33s
win+Meson test (3)	14m 11s
win+Meson test (7)	14m 18s
win+Meson test (0)	10m 53s
win+Meson test (5)	10m 1s
win+Meson test (4)	4m 53s
win test (3)	11m 29s
win test (1)	8m 33s
win test (0)	12m 10s
win test (6)	7m 24s
win test (7)	5m 53s
win test (8)	6m 51s
win test (2)	6m 39s
win test (9)	5m 28s
win test (5)	10m 32s
win test (4)	6m 35s
	7h 44m 18s

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