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_pyx3.vim b/src/testdir/test_pyx3.vim
index 1b7bdae..5dfa6cd 100644
--- a/src/testdir/test_pyx3.vim
+++ b/src/testdir/test_pyx3.vim
@@ -1,9 +1,8 @@
 " Test for pyx* commands and functions with Python 3.
 
 set pyx=3
-if !has('python3')
-  throw 'Skipped, python3 feature missing'
-endif
+source check.vim
+CheckFeature python3
 
 let s:py2pattern = '^2\.[0-7]\.\d\+'
 let s:py3pattern = '^3\.\d\+\.\d\+'