Create a test theory for QED and reorganize tests

Created by: scarlehoff

As mentioned in these two comments

https://github.com/NNPDF/nnpdf/pull/1643/files#r1146321927 https://github.com/NNPDF/nnpdf/pull/1643/files#r1155696692

the tests for the photon should be reorganized and modified so that we are testing something a bit closer to the actual usage. Similar to what is done for other things (like the new cfactors).

This is related #1729 (closed) since also Theory 399 needs to be updated to have a small eko to test the info file. I'll coordinate with @niclaurenti to create the new theories and update the tests.