test: Add testcase covering discard functionality

Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>
imgbase: Remove some unneeded prints

Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>
test: Move storage related test into module

Storage related tests are now in a module, that way we can be more
selective when running sanity checks.

Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>