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_termcodes.vim b/src/testdir/test_termcodes.vim
index 9d7f6b5..c6327db 100644
--- a/src/testdir/test_termcodes.vim
+++ b/src/testdir/test_termcodes.vim
@@ -2,10 +2,10 @@
 
 " This only works for Unix in a terminal
 if has('gui_running')
-  throw 'Skipped, does not work in the GUI'
+  throw 'Skipped: does not work in the GUI'
 endif
 if !has('unix')
-  throw 'Skipped, not on Unix'
+  throw 'Skipped: not on Unix'
 endif
 
 source shared.vim