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.

test/functional/legacy/autochdir_spec.lua

@@ -0,0 +1,26 @@
+local lfs = require('lfs')
+local helpers = require('test.functional.helpers')(after_each)
+local clear, eq = helpers.clear, helpers.eq
+local eval, execute = helpers.eval, helpers.execute
+
+describe('autochdir behavior', function()
+  local dir = 'Xtest-functional-legacy-autochdir'
+
+  before_each(function()
+    lfs.mkdir(dir)
+    clear()
+  end)
+
+  after_each(function()
+    helpers.rmdir(dir)
+  end)
+
+  -- Tests vim/vim/777 without test_autochdir().
+  it('sets filename', function()
+    execute('set acd')
+    execute('new')
+    execute('w '..dir..'/Xtest')
+    eq('Xtest', eval("expand('%')"))
+    eq(dir, eval([[substitute(getcwd(), '.*[/\\]\(\k*\)', '\1', '')]]))
+  end)
+end)