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.

test/unit/os/shell_spec.lua

@@ -25,7 +25,6 @@ local NULL = ffi.cast('void *', 0)
 
 describe('shell functions', function()
   setup(function()
-    cimported.event_init()
     -- os_system() can't work when the p_sh and p_shcf variables are unset
     cimported.p_sh = to_cstr('/bin/bash')
     cimported.p_shcf = to_cstr('-c')