Remove indeterminism in oldfiles_spec.lua

If Nvim is in a "Press ENTER..." screen before the `get_vvar()` call, the test
will hang.

test/functional/ex_cmds/oldfiles_spec.lua

@@ -62,6 +62,9 @@ describe(':oldfiles!', function()
     _clear()
     execute('rshada! ' .. shada_file)
 
+    -- Ensure nvim is out of "Press ENTER..." screen
+    feed('<cr>')
+    
     -- Ensure v:oldfiles isn't busted.  Since things happen so fast,
     -- the ordering of v:oldfiles is unstable (it uses qsort() under-the-hood).
     -- Let's verify the contents and the length of v:oldfiles before moving on.