Refactor unit tests which use Schemas

[t616178]

Currently there is a lot of repetition in our test classes for the creation and use of schemas - this should be refactored to encourage reuse.

[t616178]

This task should be tackled piecemeal after the implementation of gh-1833.

[n3101]

@t92549 @t511203 Our tests have undergone some changes over the years so is this still relevant? "Currently there is a lot of repetition in our test classes for the creation and use of schemas - this should be refactored to encourage reuse."

[t92549]

@n3101 I think this is still relevant. This kind of relates to a whole load of ideas described in https://github.com/gchq/Gaffer/issues/1833. Ideas which I have been toying with recently and I didn't realise were documented anywhere by someone else. As described, it fits into a piece of work that starts with #1867

[GCHQDeveloper314]

An additional problem with use of schemas is that various unit tests in Graph and Store make the assumption that the schema used to create a store will be the same as the schema returned by getSchema when it is actually getOriginalSchema which is guaranteed to return this. The getOriginalSchema method only works in combination with a Graph and remains null if only a Store has been created.

Therefore there are unit tests which use mocking to create behaviour which differs from the actual behaviour. This should be fixed as part of any refactor.