Unit test for HTTP 500 Internal Server Error

Currently an internal server error can be triggered by accessing an
entry belonging to a redirect loop. The ZIM file (test/data/poor.zim)
containing such a loop was copied from openzim/zim-tools repository
(test/data/zimfiles/poor.zim).
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f