vim-patch:8.1.0025: no test for the undofile() function

Problem:    No test for the undofile() function.
Solution:   Add test. (Dominique Pelle, closes vim/vim#2958)
e5fa11186f

src/nvim/testdir/test_undo.vim

@@ -357,3 +357,35 @@ func Test_undo_append()
   norm o
   quit
 endfunc
+
+funct Test_undofile()
+  " Test undofile() without setting 'undodir'.
+  if has('persistent_undo')
+    call assert_equal(fnamemodify('.Xundofoo.un~', ':p'), undofile('Xundofoo'))
+  else
+    call assert_equal('', undofile('Xundofoo'))
+  endif
+  call assert_equal('', undofile(''))
+
+  " Test undofile() with 'undodir' set to to an existing directory.
+  call mkdir('Xundodir')
+  set undodir=Xundodir
+  let cwd = getcwd()
+  if has('win32')
+    " Replace windows drive such as C:... into C%...
+    let cwd = substitute(cwd, '^\([A-Z]\):', '\1%', 'g')
+  endif
+  let cwd = substitute(cwd . '/Xundofoo', '/', '%', 'g')
+  if has('persistent_undo')
+    call assert_equal('Xundodir/' . cwd, undofile('Xundofoo'))
+  else
+    call assert_equal('', undofile('Xundofoo'))
+  endif
+  call assert_equal('', undofile(''))
+  call delete('Xundodir', 'd')
+
+  " Test undofile() with 'undodir' set to a non-existing directory.
+  call assert_equal('', undofile('Xundofoo'))
+
+  set undodir&
+endfunc