test/unit: ensure event_init()

Closes #4635
References #4630
References https://github.com/neovim/neovim/pull/4070#discussion_r50626558

test/unit/helpers.lua

@@ -140,6 +140,7 @@ do
   local time = cimport('./src/nvim/os/time.h')
   time.time_init()
   main.early_init()
+  main.event_init()
 end
 
 -- C constants.