Added get selected value from SwiftUI.Picker component

[tobywoollaston]

With TextField ViewType I am able to get the current input value with .input()

I wanted to be able get the currently selected value from a Picker ViewType without the need to inject the binding attribute or have public getters on the view.

This commit allows the selectedValue() method of InpsectableView<ViewType.Picker> to get the current value that is a binding.

let binding = Binding<String>(wrappedValue: "First Option")
let view = Picker(selection: binding, label: Text("Title")) {
        Text("First Option").tag(0)
        Text("Second Option").tag(1)
}
try view.inspect().picker().select(value: "Second Option")
        
XCTAssertEqual("Second Option", try view.inspect().picker().selectedValue())

[nalexn]

Thanks for the PR @tobywoollaston ! I tweaked the API a bit to align better with the other functions