tests: loaders: ZIP: Two more testcases.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
loaders: ZIP: Fix loading from mixed content zips

* Fix loading from ZIP files with mixed content
  (the loader wrongy aborted on unknown stored
   file due to wrong error propagation)

* Prepare for end-of-archive detection

Signed-off-by: Cyril Hrubis <metan@ucw.cz>