Based on the feedback from Tomáš in [0] this commit adds tests
that ensure consistent behavior between the python and the json
loader.
It's not 100% because the python is extremly leaniant and does
not even check if the required pieces of the json are there.
I.e. it will load a module without a SCHEMA or SCHEMA_2 variable
and the json loader code will warn about the issue but not
raise an error.
Fwiw, I have no strong opinion here but I do lean slightly towards
staying close to the original code (but both approaches of failing
with an exectption and continue with a warning have good arguments).
[0] https://github.com/osbuild/osbuild/pull/1618#discussion_r1521141148
a7b4565445