Add additional tests around merging schemas with intersections

[t92549]

2449 Allowed merging of schemas that overlap each other. More tests should be done around edge cases with aggregation to confirm we are happy with the behaviour before release 1.18.0.

Examples tests:

• Test aggregation when graph1 has prop1 but graph2 has prop1 and prop2. Currently, aggregation is not tested.
• Test this aggregation scenario with an aggregation method which fails when one property is empty.
• Test his scenario when graph1 also has a count but graph2 has two counts. How do these counts differ? Is it counter intuitive?

[t92549]

Also improve variable names such as a: https://github.com/gchq/Gaffer/blob/768f83f392dd57ff4d27c537fe3612a866db26b7/store-implementation/federated-store/src/test/java/uk/gov/gchq/gaffer/federatedstore/FederatedStoreSchemaTest.java#L131