Currently, for ApiTestCase and all subclasses
* the global `$wgUser` is set via `setMwGlobals()` (set to the test sysop, `MediaWikiIntegrationTestCase::$users['sysop']`)
* when `doApiRequest()` is called with a performer, this results in `$wgUser` being updated to the associated User object
* when `doApiRequest()` is called without a performer, the performer to use is retrieved from `$wgUser`
However, no deployed extensions appear to need these. Accordingly, we should
* stop setting up `$wgUser` via `setMwGlobals()`
* stop updating `$wgUser` if `doApiRequest()` is called with a performer
* stop reading from `$wgUser` if `doApiRequest()` is called without a performer, instead using the sysop directly
If any (non-deployed) extension was relying on $wgUser instead of the context user, *and* in the tests the $wgUser was set to a different user, this will probably cause tests to fail, prompting them to fix their code. It should have no impact on production usage