[WIP][Translation] Clear libxml errors after parsing xliff file

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

If libxml_use_internal_errors is set to `true` before parsing xliff file, the libxml errors are noit cleared correctly. An error `Validation failed: no DTD found !` occurs in libxml errors and it's available outside the xliff parser.

This and the following two cases don't need asserting errors are empty. It is a duplicataction of the preceeding test cases and not really related to these ones.