mediawiki.hook: Move mw.hook tests to new mediawiki.base.test.js file

mediawiki.hook: Move mw.hook tests to new mediawiki.base.test.js file

Follows-up 3801e54c29a.

Also:

* Split the test into smaller more dedicated tests.
* Make minor changes so as to only assert values in the tests'
  outer scope. Assertions within callbacks are an anti-pattern
  that is fragile and can easily miss or mask problems. Using a
  single state observer that is modified by the callbacks makes
  for strict assertions with no implied or untested behaviour.
  Callbacks running in a different order or a different number of
  times now cause assertion failures - instead of causing the
  assertion to not be run, or to be run multiple times, which
  would pass.

Bug: T192623
Change-Id: Ice1560b754f8df29ca583eea19f559020fafaf12