test: disable full image building in generation.sh

The purpose of the test is to check that the dnf-json socket can be
started automatically when running the test case generator while the
service or socket isn't enabled/started.

dnf-json will still be used to depsolve the packages and create the
manifest even if the image is not built.

test/cases/generation.sh

@@ -37,4 +37,5 @@ sudo ./tools/test-case-generators/generate-test-cases\
     --arch x86_64\
     --distro rhel-8\
     --image-type qcow2\
-    --store "$WORKDIR"
+    --store "$WORKDIR"\
+    --keep-image-info