tests: support msg with global_helpers.ok (#10820)

Ref: https://github.com/neovim/neovim/pull/10768#discussion_r315904175

Co-Authored-By: Justin M. Keyes <justinkz@gmail.com>

test/functional/eval/timer_spec.lua

@@ -94,7 +94,8 @@ describe('timers', function()
     command("call timer_start(5, 'MyHandler', {'repeat': -1})")
     nvim_async("command", "let g:val = 0 | let g:c = getchar()")
     retry(nil, nil, function()
-      ok(eval("g:val") >= 2)
+      local val = eval("g:val")
+      ok(val >= 2, "expected >= 2, got: "..tostring(val))
       eq(0, eval("getchar(1)"))
     end)
     feed("c")