vim-patch:8.2.3108: test for remote_foreground() fails

Problem:    Test for remote_foreground() fails. (Elimar Riesebieter)
Solution:   Check that $DISPLAY is set. (Christian Brabandt)

d6fa7bd5b9

Co-authored-by: Bram Moolenaar <Bram@vim.org>

src/nvim/testdir/check.vim

@@ -123,6 +123,14 @@ func CheckCanRunGui()
   endif
 endfunc
 
+" Command to Check for an environment variable
+command -nargs=1 CheckEnv call CheckEnv(<f-args>)
+func CheckEnv(name)
+  if empty('$' .. a:name)
+    throw 'Skipped: Environment variable ' .. a:name .. ' is not set'
+  endif
+endfunc
+
 " Command to check that we are using the GUI
 command CheckGui call CheckGui()
 func CheckGui()

src/nvim/testdir/test_clientserver.vim

@@ -13,9 +13,7 @@ source shared.vim
 
 func Check_X11_Connection()
   if has('x11')
-    if empty($DISPLAY)
-      throw 'Skipped: $DISPLAY is not set'
-    endif
+    CheckEnv DISPLAY
     try
       call remote_send('xxx', '')
     catch