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_reltime.vim b/src/testdir/test_reltime.vim
index 1876539..404ea3f 100644
--- a/src/testdir/test_reltime.vim
+++ b/src/testdir/test_reltime.vim
@@ -1,11 +1,8 @@
 " Tests for reltime()
 
-if !has('reltime')
-  throw 'Skipped, reltime feature missing'
-endif
-if !has('float')
-  throw 'Skipped, float feature missing'
-endif
+source check.vim
+CheckFeature reltime
+CheckFeature float
 
 func Test_reltime()
   let now = reltime()