i18n: support separate test suites for different natural languages

[pdawyndt]

The simplest way to have automated assessment done by TESTed, is to use a separate test suite for each natural language supported by an exercises. Supporting this onlys require a minor change to TESTed. The metadata passed to TESTed specifies the "natural language" and the "path of the test suite". Instead of passing a single path to a test suite, we could also provide a map from natural language to a specific test suite for that language.

Note: Supporting i18n this only makes sense in case the test suites for the individual natural languages are completely disjoint. Otherwise this would yield a lot of duplication in the test suites.