t/fiotestlib: improve JSON decoding #788

Job	Run time
build (linux-gcc)	2m 10s
build-containers (debian, debian, bookworm, x86_64)	1m 24s
build (linux-clang)	2m 5s
build-containers (fedora, fedora, 40, x86_64)	2m 22s
build (macos)	1m 56s
build-containers (alma, almalinux, 9, x86_64)	2m 46s
build (linux-i686-gcc)	1m 18s
build-containers (oracle, oraclelinux, 9, x86_64)	2m 26s
build (android)	37s
build-containers (rocky, rockylinux, 9, x86_64)	2m 45s
build (windows-cygwin-64)	5m 31s
build-containers (ubuntu, ubuntu, noble, i686)	1m 20s
build (windows-cygwin-32)	4m 26s
build-containers (ubuntu, ubuntu, noble, x86_64)	2m 31s
build (windows-msys2-64)	3m 16s
build (android-recovery, ubuntu-22.04, aarch64-linux-android32)	38s
	37m 31s

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