Check that the commonmark reader handles the `ascii_identifiers`
extension properly.

```
% pandoc -f commonmark+gfm_auto_identifiers+ascii_identifiers -t native
# non ascii ⚠️ räksmörgås
^D
[Header 1 ("non-ascii--raksmorgas",[],[]) [Str "non",Space,Str "ascii",Space,Str "\9888\65039",Space,Str "r\228ksm\246rg\229s"]]
```

```
% pandoc -f commonmark+gfm_auto_identifiers-ascii_identifiers -t native
# non ascii ⚠️ räksmörgås
^D
[Header 1 ("non-ascii-\65039-r\228ksm\246rg\229s",[],[]) [Str "non",Space,Str "ascii",Space,Str "\9888\65039",Space,Str "r\228ksm\246rg\229s"]]
```

`gfm` should have `ascii_identifiers` enabled by default.

```
% pandoc -f gfm -t native
# non ascii ⚠️ räksmörgås
^D
[Header 1 ("non-ascii--raksmorgas",[],[]) [Str "non",Space,Str "ascii",Space,Str "\9888\65039",Space,Str "r\228ksm\246rg\229s"]]
```