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_arabic.vim b/src/testdir/test_arabic.vim
index 450c6f9..b679ec4 100644
--- a/src/testdir/test_arabic.vim
+++ b/src/testdir/test_arabic.vim
@@ -2,9 +2,8 @@
 " NOTE: This just checks if the code works. If you know Arabic please add
 " functional tests that check the shaping works with real text.
 
-if !has('arabic')
-  throw 'Skipped: arabic feature missing'
-endif
+source check.vim
+CheckFeature arabic
 
 source view_util.vim