extend test_kiwi::test_kiwi_create_image for non-x86_64 builds

[dirkmueller]

currently the tests is hardcoding things for x86_64 build and has no chance to pass otherwise.

see the x86 in

kiwi_cmd = "kiwi-ng system build --description kiwi/build-tests/x86/leap/test-image-disk --set-repo obs://openSUSE:Leap:15.5/standard --target-dir /tmp/myimage"

and

"/tmp/myimage/kiwi-test-image-disk.x86_64-*.raw"

in result-files. the test needs to be extended to handle non-x86_64 and then the skip-if introduced in #562 should be removed again.