Add test for sources within included files

[imphil]

Reproducer for #342, please have a look at this issue. This PR is not intended for merging.

[packit-as-a-service]

We were not able to find or create Copr project packit/packit-specfile-341 specified in the config with the following error:

Cannot create a new Copr project (owner=packit project=packit-specfile-341 chroots=['epel-8-x86_64']): Copr: 'packit/packit-specfile-341' already exists

---

Please check your configuration for:

1. typos in owner and project name (groups need to be prefixed with @)
2. whether the project name doesn't contain not allowed characters (only letters, digits, underscores, dashes and dots must be used)
3. whether the project itself exists (Packit creates projects only in its own namespace)
4. whether Packit is allowed to build in your Copr project
5. whether your Copr project/group is not private

[softwarefactory-project-zuul]

Build failed.
https://softwarefactory-project.io/zuul/t/packit-service/buildset/977ba12bb0f4434bb9753fd8dd6644de

❌ pre-commit FAILURE in 2m 02s
❌ specfile-tests-rpm-deps FAILURE in 1m 11s
❌ specfile-tests-pip-deps FAILURE in 1m 05s