Improved SPIDER test coverage

[radarhere]

At https://app.codecov.io/gh/python-pillow/Pillow/blob/main/Tests%2Ftest_file_spider.py, you can see that the last line of

Pillow/Tests/test_file_spider.py

Lines 154 to 157 in 128f3f4

	def test_nonstack_dos() -> None:
	with Image.open(TEST_FILE) as im:
	for i, frame in enumerate(ImageSequence.Iterator(im)):
	assert i <= 1, "Non-stack DOS file test failed"

is not covered.

It is testing

Pillow/src/PIL/SpiderImagePlugin.py

Lines 175 to 178 in 128f3f4

	def seek(self, frame: int) -> None:
	if self.istack == 0:
	msg = "attempt to seek in a non-stack file"
	raise EOFError(msg)

This PR changes the test to directly call seek(), so all test lines are run, and test coverage is improved.