Add missing test file.

src/testdir/test_ex_undo.vim

@@ -0,0 +1,19 @@
+" Tests for :undo
+
+func Test_ex_undo()
+  new ex-undo
+  setlocal ul=10
+  exe "normal ione\n\<Esc>"
+  setlocal ul=10
+  exe "normal itwo\n\<Esc>"
+  setlocal ul=10
+  exe "normal ithree\n\<Esc>"
+  call assert_equal(4, line('$'))
+  undo
+  call assert_equal(3, line('$'))
+  undo 1
+  call assert_equal(2, line('$'))
+  undo 0
+  call assert_equal(1, line('$'))
+  quit!
+endfunc