Document how to mock user input

[alice-i-cecile]

How can Bevy's documentation be improved?

Simulating user input for use in tests is very important, but it's not always obvious how to do so.

We should document how to do this in two different places:

1. The root tests directory, as part of an integration test.
2. In a doc example in each of the input methods.

A user provided the following example for touch input, which should be helpful to get started.

pub fn mock_touch(
    mouse: Res<Input<MouseButton>>,
    windows: Res<Windows>,
    mut touch_events: EventWriter<TouchInput>,
) {
    let window = windows.get_primary().unwrap();
    let touch_phase = if mouse.just_pressed(MouseButton::Left) {
        Some(TouchPhase::Started)
    } else if mouse.just_released(MouseButton::Left) {
        Some(TouchPhase::Ended)
    } else if mouse.pressed(MouseButton::Left) {
        Some(TouchPhase::Moved)
    } else {
        None
    };
    if let (Some(phase), Some(cursor_pos)) = (touch_phase, window.cursor_position()) {
        touch_events.send(TouchInput {
            phase: phase,
            position: cursor_pos,
            force: None,
            id: 0,
        })
    }
    ```

[mockersf]

There is already an example in the tests for keyboard input:

bevy/tests/how_to_test_systems.rs

Lines 79 to 81 in d8d3fdf

	let mut input = Input::<KeyCode>::default();
	input.press(KeyCode::Space);
	world.insert_resource(input);

[alice-i-cecile]

Closing as redundant to #2896.