testsuite fails with liblouis 3.28.0

Hello,
Since the release of liblouis 3.28.0, liblouisutdml's testsuite fails:

```
FAIL: test_mathml_woluwe/test_080.test
FAIL: test_mathml_woluwe/test_106.test
FAIL: test_mathml_woluwe/test_107.test
FAIL: test_mathml_woluwe/test_113.test
FAIL: test_mathml_woluwe/test_114.test
```

Samuel