t/lib-httpd: make CGI test helpers concurrency-safe #28917

Job	Run time
config	6s
fuzz smoke test	1m 23s
win build	5m 11s
win+Meson build	4m 24s
rust-analysis	1m 37s
osx-reftable (macos-14)	28m 34s
sparse	3m 42s
static-analysis	5m 49s
documentation	7m 11s
osx-gcc (macos-14)	40m 17s
osx-meson (macos-14)	38m 38s
osx-clang (macos-14)	43m 40s
linux-musl-meson (alpine:latest)	19m 14s
linux-TEST-vars (ubuntu:20.04)	17m 10s
linux-sha256 (ubuntu:rolling)	23m 13s
linux-meson (ubuntu:rolling)	24m 17s
linux-reftable (ubuntu:rolling)	24m 34s
linux-breaking-changes (ubuntu:rolling)	29m 12s
almalinux-8 (almalinux:8)	22m 4s
debian-11 (debian:11)	22m 43s
fedora-breaking-changes-meson (fedora:latest)	23m 21s
win+VS build	-1s
linux-reftable-leaks (ubuntu:rolling)	1h 0m 46s
linux32 (i386/ubuntu:20.04)	6h 0m 3s
linux-leaks (ubuntu:rolling)	58m 52s
linux-asan-ubsan (ubuntu:rolling)	1h 12m 59s
win+VS test	-1s
win+Meson test (3)	58m 59s
win+Meson test (7)	28m 6s
win+Meson test (9)	9m 30s
win+Meson test (0)	11m 29s
win+Meson test (6)	15m 29s
win+Meson test (4)	6m 22s
win+Meson test (8)	9m 1s
win+Meson test (2)	7m 30s
win+Meson test (5)	9m 47s
win+Meson test (1)	7m 45s
win test (4)	10m 32s
win test (3)	11m 4s
win test (8)	22m 50s
win test (7)	58m 28s
win test (5)	10m 34s
win test (2)	6m 8s
win test (6)	8m 17s
win test (0)	11m 17s
win test (1)	8m 14s
win test (9)	4m 1s
	20h 54m 21s

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