Add unit tests

I'd like to have unit tests for this module *before* putting any stable release out.

@Gabor, is it possible that we can have some l10n_update_test.xx.po files in the server with a few known strings so we can properly test the download & import API?

Or I'm thinking maybe I can add a few foo strings to this module like 'Test 1 English', 'Test 2 English' to be translated to 'Test 1 Spanish', 'Test 2 Spanish'...