patch 8.1.1826: tests use hand coded feature and option checks

Problem:    Tests use hand coded feature and option checks.
Solution:   Use the commands from check.vim in more tests.
diff --git a/src/testdir/test_fold.vim b/src/testdir/test_fold.vim
index ed87007..dc58795 100644
--- a/src/testdir/test_fold.vim
+++ b/src/testdir/test_fold.vim
@@ -1,5 +1,6 @@
 " Test for folding
 
+source check.vim
 source view_util.vim
 source screendump.vim
 
@@ -707,9 +708,7 @@
 endfunc
 
 func Test_folds_with_rnu()
-  if !CanRunVimInTerminal()
-    throw 'Skipped: cannot make screendumps'
-  endif
+  CheckScreendump
 
   call writefile([
 	\ 'set fdm=marker rnu foldcolumn=2',