This method only is async in order to allow callers to wait for a process switch
triggered by the call to `loadURI` to be finished before resolving. With
DocumentChannel, we should never trigger a process switch eagerly like this
again, so we don't need any of the async behaviour here anymore.
This part is largely mechanical changes to tests, removing the `await` calls on
`loadURI`, and a follow-up part will remove the actual async logic from
`BrowserTestUtils.loadURI`.
Differential Revision: https://phabricator.services.mozilla.com/D94641
f6849f1603