test: system(): backgrounded shell command

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

runtime/doc/vim_diff.txt

@@ -108,6 +108,7 @@ Options:
 
 Commands:
   |:CheckHealth|
+  |:drop| is available on all platforms
   |:Man| is available by default, with many improvements such as completion
 
 Functions: