GD-655: Add ability to take screenshots during tests

[hatmix]

The ability to take screenshots during tests is useful in a number of ways.

• Verifying layout at various resolutions
• Determining whether a given control is visible or clipped via code can be complicated, while manual confirmation of images is quick and easy if images exist (i.e. the time consuming part is navigating the app to the correct screen to verify)
• Comparison to "known good" screenshot for when no changes are expected
• Visual regression testing

Describe the solution you'd like
Trying the normal approach to capturing the viewport in gdUnit4 returns an empty texture. It would be ideal if Godot's standard get_viewport().get_texture() returned the expected rendered view. It would be very acceptable for scene runner to have a method like scene_runner.get_viewport_texture() that emulates the behavior of get_viewport().get_texture() when running scenes normally.

Describe alternatives you've considered
This is an activity that can be performed less frequently than automated unit or functional testing, so having a special scene or an editor script to gather screenshots is sufficient, if not ideal.

[MikeSchulze]

We should provide two new functions to the SceneRunner

func capture() -> Texture2D
func store_capture(<Texture2D>, <path>) -> void

[MikeSchulze]

And provide new assert to compare screenshots.
func assert_img(<Texture2D>).is_equal(<path to saved screenshot>)

A test could look like

func test_output_scene() -> void:

	var runner := scene_runner("res://scene.tscn")
	
	# wait the scene is fully rendered
	runner.simulate_frames(10)
	
	# capture the actual screen
	var screen_shot := runner.capture()
	# verify the image with a previous captured shot
	assert_image(screen_shot).is_equal("res://test/resources/img_0815.png")
  

[hatmix]

An idea that just came to me, but is not necessary for my present use case... the ability to append screenshot(s?) to the report. Ideally, this would be attached to the test case where the screenshot was taken. If possible, this leads to a bunch of ideas about screenshots on failed assertions, etc.

[MikeSchulze]

An idea that just came to me, but is not necessary for my present use case... the ability to append screenshot(s?) to the report. Ideally, this would be attached to the test case where the screenshot was taken. If possible, this leads to a bunch of ideas about screenshots on failed assertions, etc.

Sounds good.
We can add a function like
capture_screenshot_is_failed() to the end of a UI test, this will attach a screenshot to the report when the test is failing.

[hatmix]

What runner.capture() does under headless is probably an important decision.

[MikeSchulze]

What runner.capture() does under headless is probably an important decision.

Only the image from the current view window will be grabbed.
var texture = scene.get_viewport().get_texture().get_image()