patch 8.1.1223: middle mouse click test fails without a clipboard

Problem:    Middle mouse click test fails without a clipboard.
Solution:   Check if the clipboard can be used. (Dominique Pelle, Christian
            Brabandt)  Also use WorkingClipboard() instead of checking for the
            "clipboard" feature.
diff --git a/src/testdir/test_termcodes.vim b/src/testdir/test_termcodes.vim
index af10e1d..2862a6a 100644
--- a/src/testdir/test_termcodes.vim
+++ b/src/testdir/test_termcodes.vim
@@ -5,6 +5,8 @@
   finish
 endif
 
+source shared.vim
+
 " Helper function to emit a terminal escape code.
 func TerminalEscapeCode(code_xterm, code_sgr, row, col, m)
   if &ttymouse ==# 'xterm2'
@@ -77,6 +79,10 @@
 endfunc
 
 func Test_xterm_mouse_middle_click()
+  if !WorkingClipboard()
+    throw 'Skipped: No working clipboard'
+  endif
+
   new
   let save_mouse = &mouse
   let save_term = &term