Add e2e tests, bug fixes for testIDs (#22537)

Summary:
This PR adds e2e tests for the Picker and DatePicker components.

While writing these tests, I also found and fixed two bugs where we wern't passing the `testID` down to the native components, so detox couldn't look them up. This confirms what was mentioned by rotemmiz [here](https://github.com/wix/Detox/issues/798#issuecomment-401412276)
Pull Request resolved: https://github.com/facebook/react-native/pull/22537

Reviewed By: cpojer

Differential Revision: D13371307

Pulled By: rickhanlonii

fbshipit-source-id: a4dfcdb5913645bceca0c7353328eeb9ad0f6558

Libraries/Components/DatePicker/DatePickerIOS.ios.js

@@ -148,6 +148,7 @@ class DatePickerIOS extends React.Component<Props> {
     return (
       <View style={props.style}>
         <RCTDatePickerIOS
+          testID={props.testID}
           ref={picker => {
             this._picker = picker;
           }}

Libraries/Components/Picker/PickerIOS.ios.js

@@ -47,6 +47,7 @@ type RCTPickerIOSType = Class<
       onStartShouldSetResponder: () => boolean,
       selectedIndex: number,
       style?: ?TextStyleProp,
+      testID?: ?string,
     |}>,
   >,
 >;
@@ -114,6 +115,7 @@ class PickerIOS extends React.Component<Props, State> {
           ref={picker => {
             this._picker = picker;
           }}
+          testID={this.props.testID}
           style={[styles.pickerIOS, this.props.itemStyle]}
           items={this.state.items}
           selectedIndex={this.state.selectedIndex}