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_mksession_utf8.vim b/src/testdir/test_mksession_utf8.vim
index 6bf5823..7d9a909 100644
--- a/src/testdir/test_mksession_utf8.vim
+++ b/src/testdir/test_mksession_utf8.vim
@@ -3,9 +3,8 @@
 set encoding=utf-8
 scriptencoding utf-8
 
-if !has('mksession')
-  throw 'Skipped, mksession feature missing'
-endif
+source check.vim
+CheckFeature mksession
 
 func Test_mksession_utf8()
   tabnew