po file content should be also checked in the tests

[mquinson]

in #62, we came to the conclusion that the po files that are produced during the tests are not part of the test. I kinda think that this is bad, and that we should also check that the po file looks as expected after the test.

But comparing it is not straightforward: we probably want to ignore the differences in the header and the ones about the location in the file. So I'm not completely sure of what we want to see here, and I'm seeking feedback about what you guys think of it.

Thanks, Mt.