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:

test/functional/ex_cmds/viml_system_spec.lua

@@ -1,7 +1,3 @@
--- Specs for
--- - `system()`
--- - `systemlist()`
-
 local helpers = require('test.functional.helpers')(after_each)
 local eq, clear, eval, feed, nvim =
   helpers.eq, helpers.clear, helpers.eval, helpers.feed, helpers.nvim
@@ -120,12 +116,22 @@ describe('system()', function()
     it('returns the program output', function()
       eq("echoed", eval('system("echo -n echoed")'))
     end)
+    it('to backgrounded command does not crash', function()
+      -- This is indeterminate, just exercise the codepath.
+      eval('system("echo -n echoed &")')
+      eq(2, eval("1+1"))  -- Still alive?
+    end)
   end)
 
   describe('passing input', function()
     it('returns the program output', function()
       eq("input", eval('system("cat -", "input")'))
     end)
+    it('to backgrounded command does not crash', function()
+      -- This is indeterminate, just exercise the codepath.
+      eval('system("cat - &", "input")')
+      eq(2, eval("1+1"))  -- Still alive?
+    end)
   end)
 
   describe('passing a lot of input', function()