test: add some validation for JSON doc output #2422

Job	Run time
Build slim tarball	1m 35s
aarch64-linux: with shared libraries	22m 20s
aarch64-darwin: with shared libraries	27m 1s
x86_64-darwin: with shared libraries	38m 29s
x86_64-linux: with shared libraries	27m 22s
	1h 56m 47s

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