Make unit tests truely locale independent

[matsduf]

Issue #102 document a unit test that was locale dependent. That case was fixed with PR #103, but in a way that it is still not truely locale independent.

There are at least two options to make the unit tests truely locale independent:

• Force the unit tests to run in locale C unless another locale is set on purpose.
• Do not depend on error messages or anything that could be locale dependent.