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_pyx2.vim b/src/testdir/test_pyx2.vim
index 64bd579..40e93c2 100644
--- a/src/testdir/test_pyx2.vim
+++ b/src/testdir/test_pyx2.vim
@@ -1,9 +1,8 @@
 " Test for pyx* commands and functions with Python 2.
 
 set pyx=2
-if !has('python')
-  throw 'Skipped, python feature missing'
-endif
+source check.vim
+CheckFeature python
 
 let s:py2pattern = '^2\.[0-7]\.\d\+'
 let s:py3pattern = '^3\.\d\+\.\d\+'