Import po: ignore translations starting with ";"

Also add tests for importLocalPoFile (altered the original method to optionally avoid output console).

Issue became evident after displaying the list of updated files after import from Locamotion ([#157](https://github.com/pascalchevrel/langchecker/pull/157)): I kept getting an update from ms/snippets.lang, but then no changes to the .lang file. 

Translation in the .po file [started](https://github.com/translate/mozilla-lang/commit/5a1a4fb851509ab362027e5767136362640e5523) with ";", so the system considered this an extra source string.

We can either strip the starting ";", or just ignore the translation, I choose to ignore it and display warnings during import.