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_python3.vim b/src/testdir/test_python3.vim
index e001360..7f48619 100644
--- a/src/testdir/test_python3.vim
+++ b/src/testdir/test_python3.vim
@@ -1,9 +1,8 @@
 " Test for python 3 commands.
 " TODO: move tests from test88.in here.
 
-if !has('python3')
-  throw 'Skipped, python3 feature missing'
-endif
+source check.vim
+CheckFeature python3
 
 func Test_py3do()
   " Check deleting lines does not trigger an ml_get error.