Ensure Tkinter hook is activated for getimage()

[radarhere]

Resolves #6020

When PhotoImage.paste calls "PyImagingPhoto", it catches a TclError in case the tkinter hooks have not been activated yet. If they haven't, then it calls _imagingtk.tkinit, which calls TkImaging_Init, which creates the "PyImaging" Tcl commands.

ImageTk.getimage does not handle the situation where the tkinter hooks have not been activated yet.

Pillow/src/PIL/ImageTk.py

Lines 274 to 281 in ab8125f

	def getimage(photo):
	"""Copies the contents of a PhotoImage to a PIL image memory."""
	im = Image.new("RGBA", (photo.width(), photo.height()))
	block = im.im
	photo.tk.call("PyImagingPhotoGet", photo, block.id)
	return im

So the user from the issue has reported a TclError being raised. This was missed in #3814.

As for testing this, although test_photoimage() looks like it is only testing getimage(), when PhotoImage is called with an image as the first argument, it calls paste immediately. In contrast, the test that I've modified just calls getimage(), and so when the test suite is run in reverse, this will be called first.