patch 8.2.0186: a couple of tests may fail when features are missing

Problem:    A couple of tests may fail when features are missing.
Solution:   Check for features. (Dominique Pelle, closes #5561)
diff --git a/src/testdir/test_highlight.vim b/src/testdir/test_highlight.vim
index f63abee..f718e90 100644
--- a/src/testdir/test_highlight.vim
+++ b/src/testdir/test_highlight.vim
@@ -622,6 +622,7 @@
 
 func Test_wincolor_listchars()
   CheckScreendump
+  CheckFeature conceal
 
   let lines =<< trim END
 	call setline(1, ["one","\t\tsome random text enough long to show 'extends' and 'precedes' includingnbsps, preceding tabs and trailing spaces    ","three"])