edit.c: Ensure undo sync when emulating <Esc>x #11706

After PR #8226 an unmapped META key in insert mode behaves like
ESC-<key> (:help i_META).

The behaviour does not fully match, since if <Esc>-<key> is pressed
manually then since it were pressed manually `gotchars` would be called
on the second <key> after insert-mode had already been left.

This would mean that `may_sync_undo` (called from `gotchars`) would
call `u_sync(FALSE)` on the second key (since we would be in normal
mode).

This overall means that <Meta-[something]> behaves differently with
respect to undo than <Esc>[something] when the [something] makes a
change.

As an example, under `nvim -u NONE`:
ihello<M-.>u

leaves the buffer empty, while
ihello<Esc>.u

leaves the buffer with one instance of `hello`.

- Fix by calling u_sync() manually in the new clause under
  `normalchar:` in `insert_handle_key`.
- Update test in tui_spec.lua that accidentally relied on the old behaviour.

src/nvim/edit.c

@@ -1234,6 +1234,7 @@ normalchar:
       // Unmapped ALT/META chord behaves like ESC+c. #8213
       stuffcharReadbuff(ESC);
       stuffcharReadbuff(s->c);
+      u_sync(false);
       break;
     }