t/lib-httpd: make CGI test helpers concurrency-safe #28914

Job	Run time
config	8s
fuzz smoke test	1m 14s
win build	5m 6s
win+Meson build	5m 2s
rust-analysis	1m 33s
sparse	3m 26s
static-analysis	6m 31s
osx-gcc (macos-14)	35m 17s
documentation	7m 19s
osx-clang (macos-14)	38m 3s
osx-reftable (macos-14)	28m 27s
osx-meson (macos-14)	48m 2s
linux-meson (ubuntu:rolling)	24m 11s
linux-leaks (ubuntu:rolling)	45m 50s
fedora-breaking-changes-meson (fedora:latest)	20m 14s
linux-asan-ubsan (ubuntu:rolling)	1h 2m 7s
linux-breaking-changes (ubuntu:rolling)	28m 21s
debian-11 (debian:11)	25m 48s
linux-reftable (ubuntu:rolling)	25m 4s
linux32 (i386/ubuntu:20.04)	1h 2m 19s
linux-TEST-vars (ubuntu:20.04)	24m 6s
linux-musl-meson (alpine:latest)	17m 42s
linux-reftable-leaks (ubuntu:rolling)	59m 56s
almalinux-8 (almalinux:8)	15m 25s
linux-sha256 (ubuntu:rolling)	25m 19s
win+VS build	-1s
win+VS test	-1s
win test (8)	14m 7s
win test (9)	5m 16s
win test (7)	24m 48s
win test (5)	9m 41s
win test (6)	9m 21s
win test (1)	8m 20s
win test (2)	6m 13s
win test (3)	11m 57s
win test (4)	9m 4s
win test (0)	12m 4s
win+Meson test (8)	9m 4s
win+Meson test (1)	5m 34s
win+Meson test (2)	7m 14s
win+Meson test (7)	8m 53s
win+Meson test (0)	11m 14s
win+Meson test (4)	6m 55s
win+Meson test (3)	6m 50s
win+Meson test (6)	5m 58s
win+Meson test (9)	5m 43s
win+Meson test (5)	5m 40s
	13h 20m 24s

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