Support asserting tab bar tab count

[KaneCheshire]

protocol TabBarContext {}
extension TabBarContext {

    func numberOfTabs(in tabBar: TabBar, is count: Int) {
        XCTAssertEqual(tabBar.numberOfTabs, count)
    }

}

and

extension TabBar {

    var numberOfTabs: Int {
        await(.exists, .hittable)
        return underlyingXCUIElement.buttons.count
    }

}