Refactor tests in test_image.py to avoid writes inside assets

[datumbox]

Currently some of the tests in test_image.py write inside the assets folder. Example:

vision/test/test_image.py

Lines 113 to 125 in 78159d6

	def test_write_jpeg(self):
	for img_path in get_images(IMAGE_ROOT, ".jpg"):
	data = read_file(img_path)
	img = decode_jpeg(data)
	basedir = os.path.dirname(img_path)
	filename, _ = os.path.splitext(os.path.basename(img_path))
	torch_jpeg = os.path.join(
	basedir, '{0}_torch.jpg'.format(filename))
	pil_jpeg = os.path.join(
	basedir, 'jpeg_write', '{0}_pil.jpg'.format(filename))
	write_jpeg(img, torch_jpeg, quality=75)

As per discussion here, all tests that write to files should write to temporary locations via tempfile or similar solutions. It's worth ensuring that the solution works seamlessly on Windows.