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_clientserver.vim b/src/testdir/test_clientserver.vim
index 3377f86..385ff91 100644
--- a/src/testdir/test_clientserver.vim
+++ b/src/testdir/test_clientserver.vim
@@ -1,8 +1,8 @@
 " Tests for the +clientserver feature.
 
-if !has('job') || !has('clientserver')
-  throw 'Skipped: job and/or clientserver feature missing'
-endif
+source check.vim
+CheckFeature job
+CheckFeature clientserver
 
 source shared.vim