test: add some validation for JSON doc output #2428

Job	Run time
Build slim tarball	1m 33s
x86_64-darwin: with shared libraries	38m 32s
aarch64-darwin: with shared libraries	24m 58s
x86_64-linux: with shared libraries	26m 59s
aarch64-linux: with shared libraries	22m 9s
	1h 54m 11s

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