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)
diff --git a/src/testdir/check.vim b/src/testdir/check.vim
index d6bfe00..504a192 100644
--- a/src/testdir/check.vim
+++ b/src/testdir/check.vim
@@ -136,6 +136,14 @@
   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()
diff --git a/src/testdir/test_clientserver.vim b/src/testdir/test_clientserver.vim
index 2704fb6..a088e17 100644
--- a/src/testdir/test_clientserver.vim
+++ b/src/testdir/test_clientserver.vim
@@ -13,9 +13,7 @@
 
 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
diff --git a/src/testdir/test_vim9_builtin.vim b/src/testdir/test_vim9_builtin.vim
index 38bd827..f6b5a09 100644
--- a/src/testdir/test_vim9_builtin.vim
+++ b/src/testdir/test_vim9_builtin.vim
@@ -1415,6 +1415,8 @@
   CheckFeature clientserver
   # remote_foreground() doesn't fail on MS-Windows
   CheckNotMSWindows
+  CheckEnv DISPLAY
+
   CheckDefFailure(['remote_foreground(10)'], 'E1013: Argument 1: type mismatch, expected string but got number')
   assert_fails('remote_foreground("NonExistingServer")', 'E241:')
 enddef
diff --git a/src/version.c b/src/version.c
index 060c2e7..9b1ac9a 100644
--- a/src/version.c
+++ b/src/version.c
@@ -756,6 +756,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    3108,
+/**/
     3107,
 /**/
     3106,