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_python2.vim b/src/testdir/test_python2.vim
index 5b10852..61cf959 100644
--- a/src/testdir/test_python2.vim
+++ b/src/testdir/test_python2.vim
@@ -1,9 +1,8 @@
 " Test for python 2 commands.
 " TODO: move tests from test87.in here.
 
-if !has('python')
-  throw 'Skipped, python feature missing'
-endif
+source check.vim
+CheckFeature python
 
 func Test_pydo()
   " Check deleting lines does not trigger ml_get error.