Speed up wcs test

One of the wcs tests were rather slow and ate up a significant amount of memory, too.

With this PR, running tests is faster by 4-5 mins on my laptop. I couldn't see the point why a 4000x2000 pixel image cannot be cut for the test purposes, please advise if it was essential.

Unnecessary imports and function(parts) were also removed (unless there were extra notes). I think ``test_all_world2pix()`` could be clean up further to better fit into the rest of the file (by removing the arguments and print statements, etc), but it doesn't cost any runtime so I left it untouched.

cc: @mdboom 
@bsipocz - actually a test doesn't necessarily need an assert - it can just check that the code there doesn't crash. So I think this was probably a valid test?