Add tests to ensure that overwriting on subsequent compositions does not fail

Description

While working on potential YAML optimisations, myself and Daniel came up with a use-case which we expected to fail, thankfully, it didn't, but it has gone untested thus far, so this patch is to add tests for this.

More detail about this can be found in #938 (closed).

tl;dr If we have two include files, when we compose, we compose these in reverse order. If the first one which we compose (the second include file) does not contain a list which we explicitly want to overwrite, and the second one we compose does, we should not fail on the first composite.

Closes #938 (closed)