test/helpers: add 'vim_init' helper

- Initializes some global variables.
- Necessary for the buffer tests in PR #904.

test/unit/helpers.moon

@@ -97,6 +97,17 @@ cstr = ffi.typeof 'char[?]'
 to_cstr = (string) ->
   cstr (string.len string) + 1, string
 
+export vim_init_called
+-- initialize some global variables, this is still necessary to unit test
+-- functions that rely on global state.
+vim_init = ->
+  if vim_init_called ~= nil
+    return
+  -- import os_unix.h for mch_early_init(), which initializes some globals
+  os = cimport './src/nvim/os_unix.h'
+  os.mch_early_init!
+  vim_init_called = true
+
 return {
   cimport: cimport
   cppimport: cppimport
@@ -107,4 +118,5 @@ return {
   lib: libnvim
   cstr: cstr
   to_cstr: to_cstr
+  vim_init: vim_init
 }