Attempt to fix the flakyness seen with tests

[lukebarnard1]

Specifically:

JS 2.1.1 (Linux 0.0.0) joining a room over federation should not get stuck at a spinner FAILED
	Did not find exactly one match (found: 0) for componentType:function (props, context, updater) {

actually meant that the room directory wasn't displayed - probably because the dispatch view_room_directory ended up on another tick of the event loop, meaning that the directory wasn't displayedi. The fix attempted in ths commit is to use this._setPage instead to view the directory. This uses setState to set the screen to the directory, so I'm not entirely convinced this will solve the problem (as setState may also end up doing things on another tick.

and

JS 2.1.1 (Linux 0.0.0) loading: MatrixClient rehydrated from stored credentials: shows a room view if we followed a room link FAILED
	MatrixChat still not ready after 5 tries
	awaitRoomView@/home/travis/build/vector-im/riot-web/test/all-tests.js:201363:90

was happening probably because in the handler for the sync event in MatrixChat (around line 840), there was one case in which the ready state may not be true (causing all 5 attempts to fail), and this case relied on starting_room_alias_payload. This starting_room_alias_payload is now redundant because of initialScreenAfterLogin, which was added recently.

[lukebarnard1]

Seeing how this now fails, I realise that the tests expect MatrixChat itself to be selecting the correct screen at the right time based on window.location, but if login is involved, this is done via initialScreenAfterLogin, which is not set by any of the tests.