test: Extract code to spawn nvim into the "spawn" helper function

This is can be used for spawning nvim outside a test context. Also refactor
screen.lua to use this function when loading the color map(It is better because
the GDB/VALGRIND environment variables are ignored)

test/functional/ui/screen.lua

@@ -123,16 +123,26 @@ if os.getenv('CI_TARGET') then
   default_screen_timeout = default_screen_timeout * 3
 end
 
-local colors = request('vim_get_color_map')
-local colornames = {}
-for name, rgb in pairs(colors) do
+do
+  local spawn, nvim_prog = helpers.spawn, helpers.nvim_prog
+  local session = spawn({nvim_prog, '-u', 'NONE', '-N', '--embed'})
+  local status, rv = session:request('vim_get_color_map')
+  if not status then
+    print('failed to get color map')
+    os.exit(1)
+  end
+  local colors = rv
+  local colornames = {}
+  for name, rgb in pairs(colors) do
     -- we disregard the case that colornames might not be unique, as
     -- this is just a helper to get any canonical name of a color
     colornames[rgb] = name
+  end
+  session:exit(0)
+  Screen.colors = colors
+  Screen.colornames = colornames
 end
 
-Screen.colors = colors
-
 function Screen.debug(command)
   if not command then
     command = 'pynvim -n -c '
@@ -497,8 +507,8 @@ function pprint_attrs(attrs)
     for f, v in pairs(attrs) do
       local desc = tostring(v)
       if f == "foreground" or f == "background" then
-        if colornames[v] ~= nil then
-          desc = "Screen.colors."..colornames[v]
+        if Screen.colornames[v] ~= nil then
+          desc = "Screen.colors."..Screen.colornames[v]
         end
       end
       table.insert(items, f.." = "..desc)