test/oldtest: restore test_autochdir.vim

We have a ported Lua version, but we should also keep the Vim version
around to make merging easier.

src/nvim/testdir/test_autochdir.vim

@@ -0,0 +1,19 @@
+" Test 'autochdir' behavior
+
+if !exists("+autochdir")
+  finish
+endif
+
+func Test_set_filename()
+  let cwd = getcwd()
+  call test_autochdir()
+  set acd
+  new
+  w samples/Xtest
+  call assert_equal("Xtest", expand('%'))
+  call assert_equal("samples", substitute(getcwd(), '.*/\(\k*\)', '\1', ''))
+  bwipe!
+  set noacd
+  exe 'cd ' . cwd
+  call delete('samples/Xtest')
+endfunc