Use fixture to re-open image for each JPEG2000 test

[radarhere]

At the start of test_file_jpeg2k.py, an image is opened.

Pillow/Tests/test_file_jpeg2k.py

Lines 32 to 33 in d59b169

	test_card = Image.open("Tests/images/test-card.png")
	test_card.load()

It is then used throughout the tests.

Pillow/Tests/test_file_jpeg2k.py

Lines 103 to 110 in d59b169

	def test_lossless_rt() -> None:
	im = roundtrip(test_card)
	assert_image_equal(im, test_card)
	def test_lossy_rt() -> None:
	im = roundtrip(test_card, quality_layers=[20])
	assert_image_similar(im, test_card, 2.0)

Rather than re-using the same image instance throughout the tests, and allowing it to be potentially modified in one test and affect all subsequent tests, this PR uses a fixture to re-open it each time, following the style of test_image_resize.py.