patch 8.1.1544: some balloon tests don't run when they can

Problem:    Some balloon tests don't run when they can.
Solution:   Split GUI balloon tests off into a separate file. (Ozaki Kiichi,
            closes #4538)  Change the feature check into a command for
            consistency.
diff --git a/src/testdir/test_timers.vim b/src/testdir/test_timers.vim
index 5b09011..03391d8 100644
--- a/src/testdir/test_timers.vim
+++ b/src/testdir/test_timers.vim
@@ -1,8 +1,7 @@
 " Test for timers
 
-if !has('timers')
-  throw 'Skipped, timers feature missing'
-endif
+source check.vim
+CheckFeature timers
 
 source shared.vim
 source screendump.vim