[PATCH 4 of 7] tests: splitting test-gendoc.t into per file tests

[timeless]

Martin von Zweigbergk wrote:
> Sounds useful, but what happens if they continue run test-gendoc.t?

They will no longer get warnings about actual content errors, only a
warning if they are working on a new locale (one w/o a
test-gendoc-$LOCALE.t file) or if they manage to delete / empty their
locale.

> Will we need to inform them (how?) to run test-gendoc-$LOCALE.t?

We can add the step to:
https://www.mercurial-scm.org/wiki/TranslatingMercurial#Development_process