mediawiki.api.test: Use sinon sandbox for unit tests

Make the unit tests faster and more standalone:

* Don't make a request to the actual API, instead provide the
  response via the fake server and purely test the mw.Api interface.

* No need for promise aggregration since the process is now
  synchronous.

* No arbitrary delays for async or animations etc. as sinon
  artificially fast-forwards setTimeout, Date and others for us.

Also:

* Move assertion for API error handling to separate test.

Change-Id: I20b17f256bc5114c6c2c3185653973076c15bc02