fixed functional ex_cmds tests

test/functional/ex_cmds/digraphs_spec.lua

@@ -7,7 +7,7 @@ local Screen = require('test.functional.ui.screen')
 describe(':digraphs', function()
   local screen
   before_each(function()
-    clear()
+    clear{args_rm={'--headless'}}
     screen = Screen.new(65, 8)
     screen:set_default_attr_ids({
       [1] = {bold = true, foreground = Screen.colors.Blue1},

test/functional/ex_cmds/grep_spec.lua

@@ -3,7 +3,9 @@ local clear, feed_command, feed, ok, eval =
   helpers.clear, helpers.feed_command, helpers.feed, helpers.ok, helpers.eval
 
 describe(':grep', function()
-  before_each(clear)
+  before_each(function()
+    clear{args_rm={'--headless'}}
+  end)
 
   it('does not hang on large input #2983', function()
     if eval("executable('grep')") == 0 then