Run this test from #7920 on current master.

This demonstrates:

• The test correctly fails on Windows/AppVeyor, before env: use libuv v1.12 getenv/setenv API #7920.
• Travis/Quickbuild fail in the same way as in env: use libuv v1.12 getenv/setenv API #7920.

  [ RUN      ] :let command multibyte env var to child process #8398 #9267: FAIL
  ...uild/neovim/neovim/test/functional/eval/let_spec.lua:53: Expected objects to be the same.
  Passed in:
  (string) ''
  Expected:
  (string) 'AìaB'
  
  stack traceback:
     (tail call): ?
     ...uild/neovim/neovim/test/functional/eval/let_spec.lua:53: in function <...uild/neovim/neovim/test/functional/eval/let_spec.lua:49><Paste>