test() ignores S4-ness of columns

[MichaelChirico]

Seen during testing of #6183

test(1, 1:10, asS4(1:10)) # passes
cat(sprintf("%s; %s\n", isS4(1:10), isS4(asS4(1:10))))
# FALSE; TRUE

Not sure we should do anything about it; in particular, we're getting this behavior from base itself:

all.equal(1:10, asS4(1:10))
# [1] TRUE

And while it's easy to do isS4() ourselves on a non-recursive object, recursively checking isS4() (e.g. on all data.table columns, or on nested list inputs) seems more trouble than it's worth.

Filing here in case others feel strongly one way or the other.