tests: loaders_suite: Fixes.

* Update the negative loader testcases

  - Write 32 zero bytes to the dummy file so that
    signature could be read by the loader

  - Add check malloc flag

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

* Set errno to ENOSYS when signature loader was not found

* close file if image signature wasn't read
  (fixes memleak)

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