Refactor usage of include_examples (#3517)

This ensures that when using `include_examples`, any variables are
placed within a context, ensuring that they don't leak outside the
current context and cause other tests to fail/pass.

This is necessary because `include_examples` adds the examples directly
in to the current context, whereas using `it_behaves_like` creates its
own nested context first.

Best reviewed [ignoring white space
changes](https://github.com/nhsuk/manage-vaccinations-in-schools/pull/3517/files?diff=unified&w=1).

Author: thomasleese