tests: test_images.sh still alters host config if KATA_DEV_MODE set

[crobinso]

tests/test_images.sh has some KATA_DEV_MODE awareness, but it's fairly minimal: if run locally the script will still overwrite /etc/kata-containers/configuration.toml and attempt to alter host docker service config, among other things.

I will send a series shortly that drops all host config editing if KATA_DEV_MODE is set. This leaves us no way to actually runtime test the generated appliance images though, so it makes test_images.sh essentially a build test only (which is still very useful IMO). I think this follows the intention of KATA_DEV_MODE as described in tests/README.md