TESTS: Tweak walkAST() so that validation of S7 objects also works in… #52

Job	Run time
future.batchtools (release)	2m 13s
future.callr (release)	1m 13s
doFuture (release)	2m 6s
future.apply (release)	1m 9s
furrr (release)	2m 5s
doFuture (release) multisession	1m 0s
future.apply (release) multisession	1m 1s
furrr (release) multisession	1m 34s
future.apply (release) keepWhere	1m 1s
future.apply (release) !keepWhere	3m 43s
	17m 5s

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