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_paste.vim b/src/testdir/test_paste.vim
index ee69641..a22e8e5 100644
--- a/src/testdir/test_paste.vim
+++ b/src/testdir/test_paste.vim
@@ -2,10 +2,10 @@
 
 " Bracketed paste only works with "xterm".  Not in GUI or Windows console.
 if has('win32')
-  throw 'Skipped, does not work on MS-Windows'
+  throw 'Skipped: does not work on MS-Windows'
 endif
 if has('gui_running')
-  throw 'Skipped, does not work in the GUI'
+  throw 'Skipped: does not work in the GUI'
 endif
 set term=xterm