system(): handle profiling and 'verbose' #8730

closes #8362

Vim's code calls `call_shell` directly from `get_system_output_as_rettv`
whereas in Nvim this function has been rewritten to not call `call_shell` but to call
`os_system` via `do_os_system`, losing the support for profiling and verbose.

Changing the code to call `call_shell` from `get_system_output_as_rettv`
seems to be too complicated to be worth it on the current version of the
code. So this commit duplicates the relevant code.

test/helpers.lua

@@ -54,6 +54,9 @@ end
 local function ok(res)
   return assert.is_true(res)
 end
+local function near(actual, expected, tolerance)
+  return assert.is.near(actual, expected, tolerance)
+end
 local function matches(pat, actual)
   if nil ~= string.match(actual, pat) then
     return true
@@ -694,6 +697,7 @@ local module = {
   map = map,
   matches = matches,
   mergedicts_copy = mergedicts_copy,
+  near = near,
   neq = neq,
   ok = ok,
   popen_r = popen_r,