This tested #1728, where redirecting a directory (`begin; something;
end < .`) would cause `status` to misbehave.
Unfortunately, on Illumos/OpenIndiana/SunOS, this returns a different
error (EINVAL instead of EISDIR), so we can't check that with our test harness, because
we can't redirect it.
Since it's not important that this gives the same error across
systems (and indeed we provide no way of intercepting the error!),
use an invocation test instead, because that allows different output per-uname.
See #5472.
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