Add xcDeferFulfillment to help with tests that have publishers. (#1278)

* Add xcAwaitDeferred to aid with tests that have publishers.

* Improve naming

* Address Comments

- Fix naming
- Add optional expectation message
- Use trailing closure syntax

UnitTests/Sources/HomeScreenViewModelTests.swift

@@ -87,8 +87,9 @@ class HomeScreenViewModelTests: XCTestCase {
         let room: RoomProxyMock = .init(with: .init(id: mockRoomId, displayName: "Some room"))
         room.leaveRoomClosure = { .failure(.failedLeavingRoom) }
         clientProxy.roomForIdentifierMocks[mockRoomId] = room
+        let deferred = deferFulfillment(context.$viewState.first(), message: "viewState should be published.")
         context.send(viewAction: .confirmLeaveRoom(roomIdentifier: mockRoomId))
-        await context.nextViewState()
+        try await deferred.fulfill()
         XCTAssertNotNil(context.alertInfo)
     }