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/helpers.lua

@@ -162,15 +162,20 @@ local function rawfeed(...)
   end
 end
 
+local function spawn(argv)
+  local loop = Loop.new()
+  local msgpack_stream = MsgpackStream.new(loop)
+  local async_session = AsyncSession.new(msgpack_stream)
+  local session = Session.new(async_session)
+  loop:spawn(argv)
+  return session
+end
+
 local function clear()
   if session then
     session:exit(0)
   end
-  local loop = Loop.new()
-  local msgpack_stream = MsgpackStream.new(loop)
-  local async_session = AsyncSession.new(msgpack_stream)
-  session = Session.new(async_session)
-  loop:spawn(nvim_argv)
+  session = spawn(nvim_argv)
 end
 
 local function insert(...)
@@ -275,6 +280,7 @@ clear()
 
 return {
   clear = clear,
+  spawn = spawn,
   dedent = dedent,
   source = source,
   rawfeed = rawfeed,
@@ -292,6 +298,7 @@ return {
   expect = expect,
   ok = ok,
   nvim = nvim,
+  nvim_prog = nvim_prog,
   nvim_dir = nvim_dir,
   buffer = buffer,
   window = window,

test/functional/ui/screen.lua

@@ -123,15 +123,25 @@ if os.getenv('CI_TARGET') then
   default_screen_timeout = default_screen_timeout * 3
 end
 
-local colors = request('vim_get_color_map')
+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
 
 function Screen.debug(command)
   if not command then
@@ -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)