Deliberate overriding via `save_as` metadata should be allowed, even after the
overwrite detection feature. This commit is to add tests for deliberate
overriding. As a result, the relevant tests *should fail* after this commit.
Added a page and an article, both to override a tag, with very old dates so
it limits the amount of diff in the generated pages.
Overriding feature introduced by d0e9c52410
Overwrite detection introduced by ff7410ce2a
