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_makeencoding.vim b/src/testdir/test_makeencoding.vim
index 6de3c70..09d36ea 100644
--- a/src/testdir/test_makeencoding.vim
+++ b/src/testdir/test_makeencoding.vim
@@ -4,7 +4,7 @@
 
 let s:python = PythonProg()
 if s:python == ''
-  throw 'Skipped, python program missing'
+  throw 'Skipped: python program missing'
 endif
 
 let s:script = 'test_makeencoding.py'