Test using _seek to skip frames

[radarhere]

Three formats - APNG, FLI and GIF raise an error if _seek is called to seek to frame that isn't the next one or the first one.

Pillow/src/PIL/FliImagePlugin.py

Lines 127 to 147 in 5ba72a9

	def seek(self, frame: int) -> None:
	if not self._seek_check(frame):
	return
	if frame < self.__frame:
	self._seek(0)
	for f in range(self.__frame + 1, frame + 1):
	self._seek(f)
	def _seek(self, frame: int) -> None:
	if frame == 0:
	self.__frame = -1
	self._fp.seek(self.__rewind)
	self.__offset = 128
	else:
	# ensure that the previous frame was loaded
	self.load()
	if frame != self.__frame + 1:
	msg = f"cannot seek to frame {frame}"
	raise ValueError(msg)

_seek shouldn't be called by end users, but ensuring it is called in the correct way internally would guard against bugs.