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_shortpathname.vim b/src/testdir/test_shortpathname.vim
index 564cd96..2d48f4b 100644
--- a/src/testdir/test_shortpathname.vim
+++ b/src/testdir/test_shortpathname.vim
@@ -2,7 +2,7 @@
 " Only for use on Win32 systems!
 
 if !has('win32')
-  throw 'Skipped, not on MS-Windows'
+  throw 'Skipped: not on MS-Windows'
 endif
 
 func TestIt(file, bits, expected)