patch 9.0.0607: verbose echo message test fails on Mac OS

Problem:    Verbose echo message test fails on Mac OS.
Solution:   Skip the test on Mac OS.
diff --git a/src/testdir/test_messages.vim b/src/testdir/test_messages.vim
index 486adcb..2cfe92c 100644
--- a/src/testdir/test_messages.vim
+++ b/src/testdir/test_messages.vim
@@ -339,7 +339,8 @@
 " Test verbose message before echo command
 func Test_echo_verbose_system()
   CheckRunVimInTerminal
-  CheckUnix
+  CheckUnix    " needs the "seq" command
+  CheckNotMac  " doesn't use /tmp
 
   let buf = RunVimInTerminal('', {'rows': 10})
   call term_sendkeys(buf, ":4 verbose echo system('seq 20')\<CR>")