refactor: Remove VimL function test_autochdir()

- Eliminate global test_autochdir.
- Eliminate VimL function test_autochdir()
- Use a lua test instead. Fails correctly after reverting
  0c43479979 / vim-patch:7.4.2015.

src/nvim/eval.lua

@@ -301,7 +301,6 @@ return {
     tan={args=1, func="float_op_wrapper", data="&tan"},
     tanh={args=1, func="float_op_wrapper", data="&tanh"},
     tempname={},
-    test_autochdir={},
     termopen={args={1, 2}},
     test={args=1},
     timer_start={args={2,3}},