ISTWCMS-6374: add alt text in image gallery test
Created imageGalleryAltTest which tests that images with an alt text have the alt text when added to image blocks and image gallery blocks. First created three images with unique alt texts uploaded with remote URL. Then one unique image was added to each of: image block, image gallery block set to slider, and image gallery block set to grid. Then the images were found using their alt text. Finally, all the images were deleted to avoid later conflicts with images in other tests.