These tests are essentially affirming a regression vs Vim. In Vim,
:echo system('cat - &', 'foo')
returns "foo", because Vim internally wraps the command with shell-specific
syntax to redirect the streams from /dev/null[1].
That can't work in Nvim because we use pipes directly (instead of temp files)
and don't wrap the command with shell-specific redirection syntax.
References #3529
References #5241
[1] http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_03_02
