This commit adds a small unit test that the inline source
works as expected by creating some test_data and then
checking that it ends up in the cache.
Note that this is also already tested in the tests in
osbuild/test/run/test_sources.py but there its a lot more
indirect and in the spirit of tests-as-documentation having
an explicit test here seems useful (its also quicker to
run and easier to discover).
Small followup for https://github.com/osbuild/osbuild/pull/2090
where this was discussed originally.
