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_cdo.vim b/src/testdir/test_cdo.vim
index aa2e4f1..8a6b746 100644
--- a/src/testdir/test_cdo.vim
+++ b/src/testdir/test_cdo.vim
@@ -1,8 +1,7 @@
 " Tests for the :cdo, :cfdo, :ldo and :lfdo commands
 
-if !has('quickfix')
-  throw 'Skipped: quickfix feature missing'
-endif
+source check.vim
+CheckFeature quickfix
 
 " Create the files used by the tests
 function SetUp()