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/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