[Form] Tests for #10108, label pluralization

This commit introduce a rewritten of multiple tests concerning form
labels. The StubTranslator now outputs [trans number=N] instead of
[trans] when transchoice is used. This is necessary to recognize the use
of transchoice. Unfortunately, it changes the output of all tests, so
they must be adapted even if they are not concerned.

I tought about changing [trans] for a fake <trans> element, which would
have given XPath flexibility, but since the output of the translator gets
encoded, it is not possible.

/cc @webmozart