test: add some validation for JSON doc output #2380

Job	Run time
Build slim tarball	1m 30s
aarch64-linux: with shared libraries	21m 12s
x86_64-linux: with shared libraries	26m 3s
aarch64-darwin: with shared libraries	26m 49s
x86_64-darwin: with shared libraries	43m 3s
	1h 58m 37s

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