Fix EditFilter bug with creating items and properties

Too often, other extensions make the assumption that
context title is the target title (of the new content),
and then run into bugs when the EditFilter hook is triggered.
We set title / id later, after the hook is run.

This patch sets a 'fake' temporary context title
that has a valid namespace for the given entity type,
to help avoid such problems with the EditFilter hook.

With the LinkBegin hook, it is transformed into a link
for the actual special page (Special:NewItem or NewProperty)

It would be better if other extensions didn't make such
assumption, and this is just a workaround for this
recurring issue.

Added / reworked the tests to ensure they catch this.

Bug: T78645
Bug: T61788
Change-Id: Ie71ebdebd18cfd217daae6db1881951d356f88b0