patch 9.1.0872: No test for W23 message

Problem:  No test for W23 message
Solution: Check for W23 message when accessing the clipboard fails
          (after v9.1.0868)

closes: #16076

Signed-off-by: Christian Brabandt <cb@256bit.org>

src/testdir/test_registers.vim

@@ -1045,7 +1045,7 @@ func Test_insert_small_delete_replace_mode()
   bwipe!
 endfunc
 
-" Test for W23 when clipboard is not available
+" Test for W24 when clipboard support is not available
 func Test_clipboard_regs_not_working()
   CheckNotGui
   if !has("clipboard")
@@ -1057,4 +1057,29 @@ func Test_clipboard_regs_not_working()
   endif
 endfunc
 
+" Check for W23 with a Vim with clipboard support,
+" but when the connection to the X11 server does not work
+func Test_clipboard_regs_not_working2()
+  CheckNotMac
+  CheckRunVimInTerminal
+  CheckFeature clipboard
+  let display=$DISPLAY
+  unlet $DISPLAY
+  " Run in a separate Vim instance because changing 'encoding' may cause
+  " trouble for later tests.
+  let lines =<< trim END
+      unlet $DISPLAY
+      call setline(1, 'abcdefg')
+      let a=execute(':norm! "+yy')
+      call writefile([a], 'Xclipboard_result.txt')
+  END
+  call writefile(lines, 'XTest_clipboard', 'D')
+  let buf = RunVimInTerminal('-S XTest_clipboard', {})
+  call term_sendkeys(buf, "\"+yy")
+  call StopVimInTerminal(buf)
+  let result = readfile('Xclipboard_result.txt')
+  call assert_match("^\\nW23:", result[0])
+  let $DISPLAY=display
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab