Make TestXmlReader/TestXmlReaderWithoutSpaces handle translated error messages

Summary:
Linking KI18n results in Qt translations being loaded on start. And
QXmlStreamReader, used internally by KoXmlDocument, returns translated error messages.
So running in a non-en locale still testing the english strings will fail.

Checking undocumented error strings is fragile already, so relying on a certain
translation context does not make things a lot worse.

Test Plan:
Tests no longer fail with Qt translations installed and running in a non-en
locale.

Reviewers: Calligra-Devel-list, boemann

Reviewed By: boemann

Subscribers: boemann

Tags: KEXI, #calligra:_3.0

Differential Revision: https://phabricator.kde.org/D2058