Expand tests for image-type-from-file-header

Expand tests for image-type-from-file-header

* test/lisp/image-tests.el (image-tests--files): New defconst.
(image-tests--type-from-file-header): New defun.
(image-type-from-file-header-test/jpeg)
(image-type-from-file-header-test/pbm)
(image-type-from-file-header-test/png)
(image-type-from-file-header-test/svg)
(image-type-from-file-header-test/tiff)
(image-type-from-file-header-test/webp)
(image-type-from-file-header-test/xbm)
(image-type-from-file-header-test/xpm): New tests.
(image-type-from-file-header-test): Delete test.