test: set notermguicolors in tests

Set 'notermguicolors' in tests which spawn a child Nvim process to force
existing tests to use 16 colors. Also refactor the child process
invocation to make things a little bit less messy.

test/functional/terminal/buffer_spec.lua

@@ -17,7 +17,6 @@ local sleep = helpers.sleep
 local funcs = helpers.funcs
 local is_os = helpers.is_os
 local skip = helpers.skip
-local nvim_prog = helpers.nvim_prog
 
 describe(':terminal buffer', function()
   local screen
@@ -446,10 +445,13 @@ end)
 describe('terminal input', function()
   it('sends various special keys with modifiers', function()
     clear()
-    local screen = thelpers.screen_setup(
-      0,
-      string.format([=[["%s", "-u", "NONE", "-i", "NONE", "--cmd", 'colorscheme vim', "--cmd", "startinsert"]]=], nvim_prog)
-    )
+    local screen = thelpers.setup_child_nvim({
+      '-u', 'NONE',
+      '-i', 'NONE',
+      '--cmd', 'colorscheme vim',
+      '--cmd', 'set notermguicolors',
+      '--cmd', 'startinsert',
+    })
     screen:expect{grid=[[
       {1: }                                                 |
       {4:~                                                 }|