t/lib-httpd: make CGI test helpers concurrency-safe #28910

Job	Run time
config	7s
fuzz smoke test	1m 51s
win+Meson build	3m 29s
win build	5m 13s
documentation	7m 23s
sparse	3m 49s
osx-reftable (macos-14)	30m 13s
osx-clang (macos-14)	27m 3s
static-analysis	5m 58s
osx-gcc (macos-14)	33m 12s
osx-meson (macos-14)	49m 50s
rust-analysis	1m 31s
win+VS build	0s
linux-reftable (ubuntu:rolling)	17m 59s
linux-sha256 (ubuntu:rolling)	25m 21s
linux-breaking-changes (ubuntu:rolling)	24m 51s
linux-asan-ubsan (ubuntu:rolling)	1h 11m 26s
linux32 (i386/ubuntu:20.04)	6h 0m 3s
linux-TEST-vars (ubuntu:20.04)	27m 51s
almalinux-8 (almalinux:8)	21m 3s
linux-musl-meson (alpine:latest)	19m 24s
linux-reftable-leaks (ubuntu:rolling)	59m 57s
fedora-breaking-changes-meson (fedora:latest)	20m 35s
linux-meson (ubuntu:rolling)	24m 20s
linux-leaks (ubuntu:rolling)	1h 0m 22s
debian-11 (debian:11)	22m 2s
win+VS test	-1s
win+Meson test (0)	11m 29s
win+Meson test (8)	5m 47s
win+Meson test (6)	13m 48s
win+Meson test (9)	7m 41s
win+Meson test (3)	30m 27s
win+Meson test (2)	7m 50s
win+Meson test (4)	6m 53s
win+Meson test (1)	7m 34s
win+Meson test (5)	8m 29s
win+Meson test (7)	29m 3s
win test (0)	10m 45s
win test (4)	9m 50s
win test (6)	8m 7s
win test (7)	57m 53s
win test (1)	8m 50s
win test (5)	9m 20s
win test (2)	4m 53s
win test (3)	10m 12s
win test (9)	5m 28s
win test (8)	18m 47s
	19h 57m 58s

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